Iron nitride permanent magnet and technique for forming iron nitride permanent magnet

ABSTRACT

A bulk permanent magnetic material may include between about 5 volume percent and about 40 volume percent Fe 16 N 2  phase domains, a plurality of nonmagnetic atoms or molecules forming domain wall pinning sites, and a balance soft magnetic material, wherein at least some of the soft magnetic material is magnetically coupled to the Fe 16 N 2  phase domains via exchange spring coupling. In some examples, a bulk permanent magnetic material may be formed by implanting N+ ions in an iron workpiece using ion implantation to form an iron nitride workpiece, pre-annealing the iron nitride workpiece to attach the iron nitride workpiece to a substrate, and post-annealing the iron nitride workpiece to form Fe 16 N 2  phase domains within the iron nitride workpiece.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/766,101, filed Aug. 5, 2015, which claims priority to InternationalApplication No. PCT/US2014/015104, filed Feb. 6, 2014, which claimspriority to U.S. Provisional App. No. 61/762,147, filed Feb. 7, 2013,the disclosures of which are incorporated by reference in theirentirety.

This invention was made with Government support under contract numberDE-AR0000199 awarded by Department of Energy, Office of ARPA-E. TheGovernment has certain rights in this invention.

TECHNICAL FIELD

The disclosure relates to permanent magnetic materials and techniquesfor forming permanent magnetic materials.

BACKGROUND

Permanent magnets play a role in many electro-mechanical systems,including, for example, alternative energy systems. For example,permanent magnets are used in electric motors or generators, which maybe used in vehicles, wind turbines, and other alternative energymechanisms. Many permanent magnets in current use include rare earthelements, such as neodymium, which result in high energy product. Theserare earth elements are in relatively short supply, and may faceincreased prices and/or supply shortages in the future. Additionally,some permanent magnets that include rare earth elements are expensive toproduce. For example, fabrication of NdFeB magnets generally includescrushing material, compressing the material, and sintering attemperatures over 1000° C., all of which contribute to highmanufacturing costs of the magnets.

SUMMARY

The disclosure describes bulk permanent magnets that include Fe₁₆N₂ andtechniques for forming bulk permanent magnets that include Fe₁₆N₂. BulkFe₁₆N₂ permanent magnets may provide an alternative to permanent magnetsthat include a rare earth element because Fe₁₆N₂ has high saturationmagnetization and magnetic anisotropy constant. The high saturationmagnetization and magnetic anisotropy constants result in a magneticenergy product that may be higher than rare earth magnets. For example,experimental evidence gathered from thin film Fe₁₆N₂ permanent magnetssuggests that bulk Fe₁₆N₂ permanent magnets may have desirable magneticproperties, including an energy product of as high as about 134MegaGauss*Oerstads (MGOe), which is about two times the energy productof NdFeB (which has an energy product of about 60 MGOe). Additionally,iron and nitrogen are abundant elements, and thus are relativelyinexpensive and easy to procure. The high energy product of Fe₁₆N₂magnets may provide high efficiency for applications in electric motors,electric generators, and magnetic resonance imaging (MRI) magnets, amongother applications.

As used herein, a bulk material may include a smallest dimension (e.g.,height, width, or length) that is greater than about 100 nanometers(nm). In some examples, a bulk material may include a smallest dimensionthat is greater than about 1 micrometer (μm), or greater than about 100μm.

In one aspect, the disclosure describes a bulk permanent magneticmaterial comprising between about 5 volume percent and about 40 volumepercent Fe₆N₂ phase domains; a plurality of nonmagnetic atoms ormolecules forming domain wall pinning sites; and a balance soft magneticmaterial, wherein at least some of the soft magnetic material ismagnetically coupled to the Fe₁₆N₂ phase domains via exchange springcoupling.

In another aspect, the disclosure describes a method that includesimplanting N+ ions in an iron workpiece using ion implantation to forman iron nitride workpiece; pre-annealing the iron nitride workpiece toattach the iron nitride to a substrate; and post-annealing the ironnitride workpiece to form Fe₁₆N₂ phase domains within the iron nitrideworkpiece.

In an additional aspect, the disclosure describes a method that includesforming a plurality of workpiece including iron nitride material, eachof the plurality of workpieces including between about 5 volume percentand about 40 volume percent of Fe₁₆N₂ phase domains, introducingadditional iron or nonmagnetic material between the plurality ofworkpieces or within at least one of the plurality of workpieces of ironnitride material, and sintering together the plurality of workpieces ofiron nitride to form a bulk magnet including iron nitride with betweenabout 5 volume percent and about 40 volume percent of Fe₁₆N₂ phasedomains.

In a further aspect, the disclosure describes a method that includesforming a plurality of textured iron nitride workpieces by implanting N+ions in a textured iron workpiece using ion implantation to form atextured iron nitride workpiece comprising between about 8 atomicpercent and about 15 atomic percent N+ ions, and post-annealing thetextured iron nitride workpiece to form a volume fraction of betweenabout 5 volume percent and about 40 volume percent of Fe₁₆N₂ phasedomains within the textured iron nitride workpiece, with a balance softmagnetic material including Fe₈N, wherein at least some of the Fe₁₆N₂phase domains are magnetically coupled to at least one of the Fe₈Ndomains by exchange spring coupling. In accordance with this aspect ofthe disclosure, the method also includes introducing nonmagneticmaterial between a first workpiece of the plurality of workpieces and asecond workpiece of the plurality of workpieces or within at least oneof the plurality of workpieces of iron nitride, and sintering togetherthe plurality of workpieces of iron nitride to form a bulk magnetincluding iron nitride with between about 5 volume percent and about 40volume percent of Fe₁₆N₂ phase domains, wherein the nonmagnetic materialforms domain wall pinning sites within the bulk magnet.

In another aspect, the disclosure describes a method that includesforming a plurality of textured iron nitride workpieces by mixingnitrogen in molten iron to result in a concentration of nitrogen atomsin the molten iron between about 8 atomic percent and about 15 atomicpercent, fast belt casting the molten iron to form a textured ironnitride workpiece, and post-annealing the textured iron nitrideworkpiece to form a volume fraction of between about 5 volume percentand about 40 volume percent of Fe₁₆N₂ phase domains within the texturediron nitride workpiece, with a balance soft magnetic material includingFe₈N, wherein at least some of the Fe₁₆N₂ phase domains are magneticallycoupled to at least one of the Fe₈N domains by exchange spring coupling.In accordance with this aspect of the disclosure, the method alsoincludes introducing nonmagnetic material between a first workpiece ofthe plurality of workpieces and a second workpiece of the plurality ofworkpieces or within at least one of the plurality of workpieces of ironnitride and sintering together the plurality of workpieces of ironnitride to form a bulk magnet including iron nitride with between about5 volume percent and about 40 volume percent of Fe₁₆N₂ phase domains,wherein the nonmagnetic material forms domain wall pinning sites withinthe bulk magnet.

In a further aspect, the disclosure describes a method including forminga plurality of workpieces of iron nitride material, each of theplurality of workpieces including between about 5 volume percent andabout 40 volume percent of Fe₁₆N₂ phase domains; introducing a pluralityof nonmagnetic atoms or molecules between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial; and sintering together the plurality of workpieces of ironnitride to form a bulk permanent magnetic material including ironnitride with between about 5 volume percent and about 40 volume percentof Fe₁₆N₂ phase domains, the plurality of nonmagnetic atoms or moleculesforming domain wall pinning sites, and a balance soft magnetic material,wherein at least some of the soft magnetic material is magneticallycoupled to the Fe₁₆N₂ phase domains via exchange spring coupling.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages of the disclosure will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram illustrating an example technique for forming apermanent magnetic including Fe₁₆N₂ magnetically hard domains and Fe₈Nmagnetically soft domains using ion implantation.

FIG. 2 is a line diagram illustrating the relationship between implantdepth (in Angstroms (Å)) and the implant energy (in kiloelectronvolts(keV)) of N+ ions in iron

FIG. 3 is an iron nitride phase diagram.

FIG. 4 is a scatter diagram representing the relationship between thenatural log of the diffusion coefficient and inverse temperature.

FIG. 5 is a conceptual diagram that shows eight (8) iron unit cells in astrained state with nitrogen atoms implanted in interstitial spacesbetween iron atoms.

FIG. 6 is a conceptual diagram illustrating a material having Fe₈Ndomains and Fe₁₆N₂ domains.

FIG. 7 is a flow diagram illustrating an example technique for forming abulk material including Fe₁₆N₂ magnetically hard domains exchange-springcoupled with Fe₈N magnetically soft domains, with domain wall pinningsites.

FIGS. 8A and 8B is a conceptual diagram that illustrates a technique forintroducing impurities between workpieces of material including Fe₁₆N₂magnetically hard domains exchange-spring coupled with Fe₈N magneticallysoft domains.

FIG. 9 is a flow diagram illustrating an example technique for forming abulk material including Fe₁₆N₂ magnetically hard domains exchange-springcoupled with Fe₈N magnetically soft domains, with domain wall pinningsites.

FIG. 10 is a conceptual diagram illustrating an example apparatus forfast belt casting to texture an example iron workpiece.

FIG. 11 is a conceptual diagram illustrating another example apparatuswith which an iron workpiece can be strained.

FIG. 12 is a flow diagram illustrating an example technique for forminga bulk material including Fe₁₆N₂ magnetically hard domainsexchange-spring coupled with Fe₈N magnetically soft domains, with domainwall pinning sites.

FIG. 13 is a conceptual diagram illustrating an example apparatus forfast belt casting to texture an example iron nitride workpiece.

FIG. 14 is a flow diagram illustrating an example technique for forminga bulk material including Fe₁₆N₂ magnetically hard domainsexchange-spring coupled with Fe₈N magnetically soft domains, with domainwall pinning sites.

FIG. 15 is a flow diagram illustrating an example technique for forminga bulk material including Fe₁₆N₂ magnetically hard domainsexchange-spring coupled with Fe₈N magnetically soft domains, with domainwall pinning sites.

FIG. 16 is a conceptual diagram illustrating an example apparatus withwhich an iron workpiece can be strained and exposed to nitrogen.

FIG. 17 illustrates further detail of one example of the Crucibleheating stage shown in FIG. 16.

FIG. 18 is a schematic diagram illustrating an example apparatus thatmay be used for nitridizing an iron workpiece via a urea diffusionprocess.

FIG. 19 is a line diagram representing an auger measurement of N+ ionconcentration as a function of depth in an iron foil after ionimplantation and before annealing the iron nitride foil.

FIG. 20 is a scatter diagram illustrating nitrogen concentrations as afunction of depth within an iron foil after post-annealing for differentnitrogen fluencies.

FIGS. 21A and 21B are hysteresis loops of magnetization versuscoercivity for examples of an iron nitride foil prepared using ionimplantation.

FIG. 22 is a line diagram that illustrates examples of nitrogen depthprofiles in iron foils before annealing.

FIG. 23 is a scatter diagram illustrating nitrogen concentrations as afunction of depth within an iron foil after post-annealing for differentnitrogen fluencies.

FIG. 24 is an x-ray diffraction (XRD) spectrum for an example of an ironnitride foil before and after annealing.

FIG. 25 is a hysteresis loop of magnetization versus coercivity forexamples of an iron nitride foil prepared using ion implantation.

FIG. 26 includes two line diagrams illustrating nitrogen bindingenergies before and after the annealing treatments.

FIGS. 27-29 illustrate XRD patterns collected for iron nitride samplesstrained to different levels during a post-annealing treatment.

FIGS. 30-32 are hysteresis loops of magnetization versus coercivity forexamples of iron nitride wires exposed to different strains duringannealing.

FIG. 33 is an example infrared image illustrating an iron foil directbonded to a (111) Si substrate.

FIG. 34 is a diagram illustrating example nitrogen depth profiles forthe ion implanted sample before the two-step annealing, which weremeasured by Auger Electron Spectroscopy (AES) with Ar⁺ as the in-depthmilling source.

FIG. 35 is a diagram illustrating example nitrogen depth profiles afterthe two-step annealing.

FIG. 36 is a diagram illustrating example XRD spectra for foil sampleswith different nitrogen fluencies on Si (111) substrate afterpost-annealing.

FIG. 37 is a diagram illustrating example hysteresis loops of the sampleprepared with 1×10¹⁷/cm² fluence at different stages.

FIG. 38 is a diagram illustrating an example hysteresis loop of thesample prepared with 5×10¹⁷/cm² fluence after post-annealing.

FIG. 39 is a diagram illustrating the calculated energy product of thefilm tested to obtain the results presented in FIG. 38.

FIG. 40 is an example high resolution transmission electron microscopy(HRTEM) image of the film tested to obtain the results presented inFIGS. 38 and 39.

FIG. 41 is an example image showing an x-ray diffraction pattern for thesample tested to obtain the results presented in FIG. 40.

DETAILED DESCRIPTION

The disclosure describes permanent magnets that include Fe₁₆N₂ phasedomains and techniques for forming permanent magnets that include Fe₁₆N₂phase domains. In particular, the techniques described herein are usedto form bulk permanent magnets that include Fe₁₆N₂ phase domains. Insome examples, the Fe₁₆N₂ phase domains may present in combination withother phases, such as phases of magnetically soft material. The Fe₁₆N₂phase may be magnetically coupled to the magnetically soft material byexchange spring coupling, which may effectively harden the magneticallysoft material and provide magnetic properties for the bulk materialsimilar to those of a bulk material formed of Fe₁₆N₂. The disclosurealso describes techniques for producing bulk magnets including Fe₁₆N₂phase, alone or in combination with other material phases.

Magnets that include a Fe₁₆N₂ phase may provide a relatively high energyproduct, for example, as high as about 134 MGOe when the Fe₁₆N₂permanent magnet is anisotropic. In examples in which the Fe₁₆N₂ magnetis isotropic, the energy product may be as high as about 33.5 MGOe. Theenergy product of a permanent magnetic is proportional to the product ofremanent coercivity and remanent magnetization. For comparison, theenergy product of Nd₂Fe₁₄B permanent magnet may be as high as about 60MGOe. A higher energy product can lead to increased efficiency of thepermanent magnet when used in motors, generators, or the like.

Additionally, permanent magnets that include a Fe₁₆N₂ phase may notinclude rare earth elements, which may reduce a materials cost of themagnet and may reduce an environmental impact of producing the magnet.In some examples, using the techniques described herein may also reducean environmental impact of the producing permanent magnets compared toprocesses used to make rare earth magnets, as the current techniques maynot include use or a powder phase or high temperature sintering steps.

However, Fe₁₆N₂ is a metastable phase, which competes with other stablephases of Fe—N. Hence, forming a bulk material including a high volumefraction of Fe₁₆N₂ may be difficult. The magnetic material includingFe₁₆N₂ phase domains described in this disclosure overcomes thisdifficulty by forming a magnetic material including Fe₁₆N₂ domains anddomains of other, magnetically soft materials. The magnetically hardFe₁₆N₂ domains magnetically couple to the magnetically soft materials byexchange spring coupling and effectively harden the magnetically softmaterials. To achieve exchange spring coupling throughout the volume ofthe magnetic material, the Fe₁₆N₂ domains may be distributed throughoutthe magnetic material, e.g., at a nanometer or micrometer scale.

As described herein, magnetic materials including Fe₁₆N₂ domains anddomains of other, magnetically soft materials may include a volumefraction of Fe₁₆N₂ domains of less than about 40 volume percent (vol.%). For example, the magnetically hard Fe₁₆N₂ phase may constitutebetween about 5 vol. % and about 40 vol. % of the total volume of themagnetic material, or between about 5 vol. % and about 20 vol. % of thetotal volume of the magnetic material, or between about 10 vol. % andabout 20 vol. % of the total volume of the magnetic material, or betweenabout 10 vol. % and about 15 vol. % of the total volume of the magneticmaterial, or about 10 vol. % of the total volume of the magneticmaterial, with the remainder of the volume being magnetically softmaterials. The magnetically soft materials may include, for example, Fe,FeCo, Fe₈N, or combinations thereof.

In some examples, such as when the magnetically soft material includesFe or Fe₈N, the crystallographic texture of the Fe₁₆N₂ and the Fe orFe₈N domains may be coherent. In other words, there may be a latticematch between the domains, including an aligned magnetic easy axis. Thismay facilitate efficient exchange-spring coupling between themagnetically hard Fe₁₆N₂ domains and the magnetically soft Fe or Fe₈Ndomains, particularly across phase boundaries.

In some examples, the magnetic material may be a bulk magnetic material.As used herein, the phrases “bulk magnetic material” and “bulk permanentmagnetic material” refer to magnetic materials and permanent magneticmaterials whose smallest dimension (e.g., length, height, or width) isgreater than about 100 nm. In other examples, a “bulk magnetic material”and “bulk permanent magnetic material” may have a smallest dimension(e.g., length, height, or width) that is greater than about 1 μm, orgreater than about 100 μm.

The magnetic material described herein may possess a relatively largeenergy product, such as greater than about 10 MGOe. In some examples,the magnetic material described herein may possess an energy productgreater than about 30 MGOe, greater than about 60 MGOe, between about 60MGOe and about 135 MGOe, greater than about 65 MGOe, or greater thanabout 100 MGOe. This may be achieved by include Fe₁₆N₂ phase domainsthat are exchange spring coupled to magnetically soft domains, alone orin combination with dopant elements (e.g., atoms) or compounds (e.g.,molecules) that form domain wall pinning sites within the magneticmaterial.

The disclosure also describes multiple processes for forming bulkpermanent magnets that include Fe₁₆N₂ domains. In some examples, thetechniques may include using ion implantation to implant N+ ions in acrystallographically textured iron workpiece. In other examples, the N+ions may be introduced into liquid Fe from a reactant, such as ammonia,ammonium azide, or urea, followed by casting to form a texturedworkpiece including iron and nitrogen ions.

Regardless of the method of forming, the workpiece including the ironand nitrogen ions may be annealed to form the Fe₁₆N₂ phase within theFe₈N phase. The annealing may occur at relatively low temperatures(e.g., between about 150° C. and about 250° C. or below about 214° C.)for between about 5 hours and about 100 hours to form Fe₁₆N₂ phasewithin the Fe₈N phase. During the annealing, the iron nitride materialmay be strained to facilitate conversion of the body-centered cubic(bcc) iron crystals into body-centered tetragonal (bct) iron nitridecrystals.

In some instances, multiple workpieces of the Fe₁₆N₂+Fe₈N material maybe combined, with or without introduction of magnetically soft ornonmagnetic dopant materials, and pressed together to form a bulkpermanent magnet. The magnetically soft or nonmagnetic materials may beprovided as workpieces of the material, using ion implantation into theworkpieces of Fe₁₆N₂+Fe₈N material, or using cluster implantation intothe workpieces of Fe₁₆N₂+Fe₈N material. The magnetically soft ornonmagnetic materials may produce domain wall pinning sites within thematerial, which may increase a magnetic coercivity of the material.

FIG. 1 is a flow diagram illustrating an example technique for forming apermanent magnet including Fe₁₆N₂ magnetically hard domains and Fe₈Nmagnetically soft domains using ion implantation. The technique shown inFIG. 1 is one technique for forming a permanent magnetic includingFe₁₆N₂ magnetically hard domains and Fe₈N magnetically soft domains, andmay be used alone or in combination with other processing steps to forma bulk permanent magnetic based on Fe₁₆N₂ exchange coupled with a softmagnetic material, such as Fe₈N. For example, the technique shown inFIG. 1 may be combined with other processing steps, as shown in FIGS. 9and 13, to form a bulk magnetic material.

The technique shown in FIG. 1 includes implanting N+ ions in an ironworkpiece using ion implantation (12). The iron workpiece may include aplurality of iron crystals. In some examples, the plurality of ironcrystals may have crystal axes oriented in substantially the samedirection. For example, a major surface of the iron workpiece may beparallel to the (110) surfaces of all or substantially all of the ironcrystals. In other examples, a major surface of the iron workpiece maybe parallel to another surface of all or substantially all of the ironcrystals. By using a workpiece in which all or substantially all of theiron crystals have substantially aligned crystal axes, anisotropy formedwhen forming the Fe₈N and Fe₁₆N₂ phases may be substantially aligned.

In some examples, workpieces include a dimension that is longer, e.g.,much longer, than other dimensions of the workpiece. Example workpieceswith a dimension longer than other dimensions include fibers, wires,filaments, cables, films, thick films, foils, ribbons, sheets, or thelike. In other examples, workpieces may not have a dimension that islonger than other dimensions of the workpiece. For example, workpiecescan include grains or powders, such as spheres, cylinders, flecks,flakes, regular polyhedra, irregular polyhedra, and any combinationthereof. Examples of suitable regular polyhedra include tetrahedrons,hexahedrons, octahedron, decahedron, dodecahedron and the like,non-limiting examples of which include cubes, prisms, pyramids, and thelike.

In some examples of the technique of FIG. 1, the workpiece includes afoil. The workpiece may define a thickness on the order of hundreds ofnanometers to millimeters. In some examples, the iron workpiece maydefine a thickness between about 500 nanometers (nm) and about 1millimeter (mm). The thickness of the iron workpiece may affect theparameters used for ion implantation and annealing of the workpiece, aswill be described below. The thickness of the workpiece may be measuredin a direction substantially normal to a surface of the substrate towhich the workpiece is attached.

Prior to implantation of N+ ions in the iron workpiece, the ironworkpiece may be positioned on a surface of a silicon substrate or agallium arsenide (GaAs) substrate. In some examples, the iron workpiecemay be position on the (111) surface of a (single crystal) siliconsubstrate, although any crystalline orientation may be used. In someexamples, the iron workpiece may be attached to the surface of thesubstrate at this time.

The average depth to which the N+ ions are implanted in the ironworkpiece may depend upon the energy to which the N+ ions areaccelerated. FIG. 2 is a line diagram illustrating the relationshipbetween implant depth (in Angstroms (Å)) and the implant energy (inkiloelectronvolts (keV)) of N+ ions in iron, as determined using SRIM(“The Stopping and Range of Ions in Matter,” software available fromJames F. Ziegler at www.srim.org). As shown in FIG. 2, the averageimplant depth of the N+ ions increases for increasing implant energy.Although not shown in FIG. 2, for each implant energy, N+ ions areimplanted within the iron workpiece in a range depths surrounding theaverage implant depth.

The implant energy used to implant the N+ ions may be selected based atleast in part on the thickness of the iron workpiece. The implant energyalso may be selected to implant the N+ ions without doing overlysignificant damage to the iron workpiece, including the crystal latticeof the iron crystals in the iron workpiece. For example, while higherimplant energies may allow implantation of the N+ ions at a greateraverage depth, higher implant energies may increase the damage to theiron workpiece, including damaging the crystal lattice of the ironcrystals and ablating some of the iron atoms due to the impact of the N+ions. Hence, in some examples, the implant energy may be limited to bebelow about 180 keV. In some examples, the incident angle ofimplantation may be about zero degrees (e.g., substantiallyperpendicular to the surface of the iron workpiece). In other examples,the incident angle of implantation may be adjusted to reduce latticedamage. For example, the incident angle of implantation may be betweenabout 3° and about 7° from perpendicular.

As an example, when the iron workpiece defines a thickness of about 500nm, an implant energy of about 100 keV may be used to implant the N+ions in the iron workpiece. An implant energy of about 100 keV may alsobe used to implant the N+ ions in iron workpieces of other thicknesses.In other examples, a different implant energy may be used for ironworkpieces defining a thickness of about 500 nm, and the same ordifferent implant energy may be used for workpieces defining a thicknessdifferent than 500 nm.

Additionally, the fluency of N+ ions may be selected to implant adesired dose of N+ ions within the iron workpiece. In some examples, thefluency of N+ ions may be selected to implant approximatelystoichiometric number of N+ ions within the iron workpiece. Thestoichiometric ratio of iron to nitrogen in Fe₁₆N₂ is 8:1. Thus, theapproximate number of iron atoms in the iron workpiece may bedetermined, and a number of N+ ions equal to approximately ⅛ (12.5%) ofthe iron atoms may be implanted in the iron workpiece, such as betweenabout 8 at. % and about 15 at. %. For example, an iron workpiece havingmeasurements of about 1 cm by 1 cm by 500 nm may include about 4.23×10¹⁸iron atoms. Thus, to achieve a stoichiometric ratio of iron atoms to N+ions in the iron workpiece, about 5.28×10¹⁷ N+ ions may be implanted inthe sample.

The temperature of the iron workpiece during the ion implantation alsomay be controlled. In some examples, the temperature of the ironworkpiece may be between about room temperature and about 500° C.

Once the N+ ions have been implanted in the iron workpiece (12), theiron workpiece may be subjected to a first annealing step (14), whichmay be referred to as a pre-annealing step. The pre-annealing step mayaccomplish multiple functions, including, for example, securelyattaching the iron workpiece to the substrate. As described below,secure attachment of the iron workpiece to the substrate allows thepost-annealing step to generate stress in the iron workpiece,facilitating the transformation of the crystalline structure of at leastsome of the crystals in the iron workpiece from body centered cubic(bcc) iron to body centered tetragonal (bct) iron nitride. In someexamples, the pre-annealing step also may activate the implanted N+ions, repair damage to the iron crystals' lattices due to the ionimplantation procedure, and/or remove any oxygen in the workpiece. Insome examples, the pre-annealing step may be performed at a temperaturebetween about 450° C. and about 550° C. for between about 30 minutes andabout 4 hours. As an example, the pre-annealing step may be performed ata temperature of about 500° C. for between about 30 minutes and about 1hour.

In some examples, in addition to heating the iron workpiece and thesubstrate, the pre-annealing step may include applying an external forcebetween about 0.2 gigapascals (GPa) and about 10 GPa between the ironworkpiece and the substrate. The external force may assist bonding ofthe iron workpiece and the substrate.

The atmosphere in which the pre-annealing step is performed may include,for example, nitrogen, argon, and/or hydrogen, such as a mixture ofabout 4 vol. % hydrogen, about 10 vol. % nitrogen, and about 86 vol. %argon. The composition of the atmosphere may assist with removing oxygenfrom the workpiece and cleaning surfaces of the workpiece.

Following the pre-annealing step (14), the iron workpiece includingimplanted N+ ions and the substrate may be exposed to a second annealingstep (16), which may be referred to as a post-annealing step. Thepost-annealing step may be carried out at a temperature that producesstrain in the iron workpiece due to differences in the coefficients ofthermal expansion for the substrate and the iron workpiece and thataccesses the Fe₁₆N₂ phase. Additionally, the post-annealing step allowsdiffusion of N+ ions iron crystals to form iron nitride, includingFe₁₆N₂ phase domains and Fe₈N phase domains. FIG. 3 is an iron nitridephase diagram, reproduced from E. H. Du Marchi Van Voorthuysen et al.Low-Temperature Extension of the Lehrer Diagram and the Iron-NitrogenPhase Diagram, 33A Metallurgical and Materials Transactions A 2593, 2597(August 2002). As shown in FIG. 3, annealing at relatively lowtemperatures allows transformation of partial Fe₈N disordered phase intoFe₁₆N₂ ordered phase. In some examples, the post-annealing step may becarried out at a temperature below about 250° C., such as between about120° C. and about 214° C., between about 120° C. and about 200° C.,between about 150° C. and about 200° C., or at about 150° C. Thepost-annealing step may be performed in a nitrogen (N₂) or argon (Ar)atmosphere, or in a vacuum or near-vacuum.

The temperature and duration of the post-annealing step may be selectedbased on, for example, a size of the sample and a diffusion coefficientof nitrogen atoms in iron at the post-annealing temperature. Based onthese factors, the temperature and duration may be selected to providesufficient time for nitrogen atoms to diffuse to locations within theiron workpiece to form Fe₁₆N₂ domains. FIG. 4 is a scatter diagramrepresenting the relationship between the natural log of the diffusioncoefficient and inverse temperature. Based on the relationship shown inFIG. 4, the natural logarithm of the diffusion coefficient may bedetermined for a given temperature, and the diffusion length may bedetermined for a given time using the diffusion coefficient. As anexample, at a temperature of 200° C., and an annealing duration of 40hours, the diffusion length of nitrogen in iron is about 0.5 micrometers(μm).

Additionally, the temperature and duration of the post-annealing stepmay be selected based on a desired volume fraction of Fe₁₆N₂ phasedomains in the iron nitride workpiece. For example, at a selectedtemperature, a longer post-annealing step may result in a higher volumefraction of Fe₁₆N₂. Similarly, for a given post-annealing step duration,a higher temperature may result in a higher volume fraction of Fe₁₆N₂.However, for durations above a threshold value, the additional volumefraction of Fe₁₆N₂ may be limited or eliminated, as the volume fractionof Fe₁₆N₂ reaches a relatively stable value. For example, at atemperature of about 150° C., after about 20 hours, the volume fractionof Fe₁₆N₂ reaches a stable value. The duration of the post-annealingstep may be at least about 5 hours, such as at least about 20 hours, orbetween about 5 hours and about 100 hours, or between about 20 hours andabout 100 hours, or about 40 hours.

Fe₈N and Fe₁₆N₂ have similar body-centered tetragonal (bct) crystallinestructure. However, in Fe₁₆N₂, nitrogen atoms are ordered within theiron lattice, while in Fe₈N, nitrogen atoms are randomly distributedwithin the iron lattice. FIG. 5 is a conceptual diagram that shows eight(8) iron unit cells in a strained state with nitrogen atoms implanted ininterstitial spaces between iron atoms in an Fe₁₆N₂ phase. As shown inFIG. 5, in the Fe₁₆N₂ phase, the nitrogen atoms are aligned along the(002) crystal plane. Also shown in FIG. 5, the iron nitride unit cellsare distorted such that the length of the unit cell along the <001> axisis approximately 3.14 angstroms (Å) while the length of the unit cellalong the <010> and <100> axes is approximately 2.86 Å. The iron nitrideunit cell may be referred to as a bct unit cell when in the strainedstate. When the iron nitride unit cell is in the strained state, the<001> axis may be referred to as the c-axis of the unit cell.

The post-annealing step facilitates formation of the bct crystallinestructure at least in part due to the strain exerted on the iron crystallattice as a result of differential expansion of the substrate and theiron nitride workpiece during the post-annealing step. For example, thecoefficient of thermal expansion for iron is 11.8 μm/m·K, while forsilicon it is 2.6 μm/m·K. This difference in thermal expansioncoefficients results in a compression stress substantially parallel themajor plane of the iron workpiece and a corresponding stretching forcebeing generated along the <001> crystalline direction on an ironworkpiece with an (110) face.

The post-annealing step results in formation of Fe₁₆N₂ phase domainswithin domains of Fe₈N, and other Fe and/or iron nitride compositions.FIG. 6 is a conceptual diagram illustrating a material having Fe₈Ndomains 22 and Fe₁₆N₂ domains 24. Because the iron workpiece isstructured on a nanometer scale (e.g., the sizes of Fe₈N domains 22 andFe₁₆N₂ domains 24 are on the order of nanometers), magnetic couplingbetween the magnetically hard Fe₁₆N₂ domains 24 and the magneticallysoft Fe₈N domains 22 may occur substantially throughout the workpiece.Because the Fe₁₆N₂ and Fe₈N crystals have substantially similarcrystalline structure, the material can be naturallycrystallographically coherent, meaning having an aligned easy axis,which produces anisotropy. This may facilitate exchange coupling throughphase boundaries between Fe₁₆N₂ and Fe₈N.

FIG. 7 is a flow diagram illustrating an example technique for forming abulk material including Fe₁₆N₂ magnetically hard domains exchange-springcoupled with Fe₈N magnetically soft domains, with domain wall pinningsites. FIG. 7 is a general technique, and further details of examples ofthe general technique of FIG. 7 will be described below with referenceto the techniques of FIGS. 9, 12, 14, and 15.

The technique of FIG. 7 includes forming an iron nitride materialincluding Fe₁₆N₂ phase domains (32). As described with reference to FIG.1, using ion implantation to implant N+ ions in an iron workpiece, suchas an iron foil, followed by one or more, e.g., two, annealing steps isone technique for forming an iron nitride material (or workpiece)including Fe₁₆N₂ phase. Other techniques for forming an iron nitridematerial including Fe₁₆N₂ phase domains (32) are described below withrespect to FIGS. 9, 12, 14, and 15. These techniques may include forminga textured iron workpiece, such as a textured iron sheet; implanting N+ions in the textured iron workpiece to form a textured iron nitrideworkpiece; and annealing the textured iron nitride workpiece to formFe₁₆N₂ phase domains. The techniques also may include forming a texturediron nitride workpiece, such as a textured iron nitride sheet, by mixingnitrogen in molten iron and annealing the textured iron nitrideworkpiece to form Fe₁₆N₂ phase domains. The techniques also may includeforming an iron workpiece, nitridizing the strained iron workpiece, andannealing the nitridized iron workpiece.

The technique illustrated in FIG. 7 also includes introducing additionaliron or nonmagnetic materials (34). In some examples, as described withreference to FIGS. 9 and 12, the additional iron or nonmagneticmaterials may be introduced as workpieces of the iron or nonmagneticmaterials interleaved with workpieces of the iron nitride materialincluding Fe₁₆N₂ phase domains. In other examples, as described withreference to FIG. 14, the additional iron or nonmagnetic materials maybe introduced into the iron nitride material including Fe₁₆N₂ phasedomains using ion implantation and/or cluster implantation. Examplenonmagnetic elements (e.g., atoms) or compounds (e.g., molecules) thatcan be used include Al, Cu, Ti, Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃,or combinations thereof. The iron or nonmagnetic materials may beintroduced as workpieces of material. The workpieces or Fe ornonmagnetic powders may have sizes (e.g., thicknesses or diameters)ranging from several nanometers to several hundred nanometers, and mayfunction as domain wall pinning sites after pressing process. Thosedomain wall pinning sites may enhance the coercivity of the permanentmagnet.

The technique illustrated in FIG. 7 further includes sinteringworkpieces to form a bulk magnetic material (36). An example by whichmultiple workpieces may be sintered together is illustrated in FIGS. 8Aand 8B. For example a plurality of iron nitride workpieces 42 a-42 c(which include Fe₁₆N₂ phase domains) may be interleaved with a pluralityof workpieces 44 a, 44 b that include iron or nonmagnetic materials.Although three iron nitride workpieces 42 a-42 c and two workpieces 44a, 44 b that include iron or nonmagnetic materials are shown in FIGS. 8Aand 8B, in other examples, any number of iron nitride workpieces 42 a-42c and two workpieces 44 a, 44 b that include iron or nonmagneticmaterials may be used.

Iron nitride workpieces 42 a-42 c may be arranged so the <001> axes ofthe respective iron nitride workpieces 42 a-42 c are substantiallyaligned. In examples in which the <001> axes of the respective ironnitride workpieces 42 a-42 c are substantially parallel to a long axisof the respective one of iron nitride workpieces 42 a-42 c,substantially aligning the iron nitride workpieces 42 a-42 c may includeoverlying one of iron nitride workpieces 42 a-42 c on another of ironnitride workpieces 42 a-42 c. Aligning the <001> axes of the respectiveiron nitride workpieces 42 a-42 c may provide uniaxial magneticanisotropy to magnet material 46 (FIG. 8B).

After the workpieces are interleaved as shown in FIG. 8A, the workpiecesmay be pressed together and sintered. The sintering pressure,temperature and duration may be selected to mechanically join theworkpieces while maintaining the crystal structure of the iron nitrideworkpieces 42 a-42 c, (e.g., as including the Fe₁₆N₂ phase domains).Thus, in some examples, the sintering step may be performed at arelatively low temperature. For example, the sintering temperature maybe below about 250° C., such as between about 120° C. and about 250° C.,between about 150° C. and about 250° C., between about 120° C. and about200° C., between about 150° C. and about 200° C., or about 150° C. Thesintering pressure may be between, for example, about 0.2 gigapascal(GPa) and about 10 GPa. The sintering time may be at least about 5hours, such as at least about 20 hours, or between about 5 hours andabout 100 hours, or between about 20 hours and about 100 hours, or about40 hours. The sintering time, temperature, and pressure may be affectedby the materials in workpieces 44 a, 44 b that include iron ornonmagnetic materials. The sintering may be performed in an ambientatmosphere, a nitrogen atmosphere, a vacuum, or another inertatmosphere.

After the sintering iron nitride workpieces 42 a-42 c and two workpieces44 a, 44 b that include iron or nonmagnetic materials, a bulk magneticmaterial 46 may be formed. Bulk magnetic material 46 may include bothexchange spring coupling between Fe₈N and Fe₁₆N₂ and domain wall pinningsites provided by the iron or nonmagnetic materials. In this way, bulkmagnetic material 46 may possess a desirably high energy product, whichmay approach that of pure Fe₁₆N₂ (about 134 MGOe).

Bulk magnetic material 46 may be formed by a plurality of differenttechniques which fall within the general technique described withrespect to FIG. 7. Examples of these different techniques are describedwith reference to FIGS. 9, 12, 14, and 15, and other techniques forforming bulk magnetic material 46 will be apparent based on the contentsof this disclosure.

FIG. 9 is a flow diagram illustrating an example technique for forming abulk material including Fe₁₆N₂ magnetically hard domains exchange-springcoupled with Fe₈N magnetically soft domains, with domain wall pinningsites. The technique of FIG. 9 includes forming a textured ironworkpiece (52). In some examples, the textured iron workpiece mayinclude textured iron sheets 74 a and 74 b (FIG. 10). In other examples,the textured iron workpiece may include another workpiece, such as afiber, a wire, a filament, a cable, a film, a thick film, a foil, aribbon, or the like. The textured iron workpiece may be formed using,for example, belt casting. FIG. 10 is a conceptual diagram illustratingone example apparatus 60 for fast belt casting to texture an exampleiron workpiece.

Fast belt casting apparatus 60 may include an ingot chamber 66 whichcontains molten iron ingot 62, and is heated by heating source 64, e.g.,in the form of a heating coil. In some examples, the temperature ofmolten iron ingot 62 within ingot chamber 66 may be greater than about1800 Kelvin (K; about 1526.85° C.). The pressure of the iron ingot 62within ingot chamber 66 may be between about 0.06 MPa and about 0.12MPa.

Molten iron ingot 62 flows out of ingot chamber 66 through nozzle head68 to form iron strip 70. Iron strip 70 is fed into the gap zone betweenthe surfaces of first pinch roller 72 a and second pinch roller 72 b(collectively, “pinch rollers 72”), which are rotated in oppositedirections. In some examples, the distance from nozzle head 68 to thesurfaces of pinch rollers 72 may be between about 2 mm and about 10 mm,such as about 4 mm.

In some examples, the rotation speed of first pinch roller 72 a andsecond pinch roller 72 b may vary from approximately 10 rotations perminute (rpm) to 1000 rpm, and the rotation speed of the rollers 72 maybe approximately the same. In some examples, pinch rollers 72 areactively cooled, e.g., using water cooling, which maintains the surfacesof rollers 72 at a temperature below the temperature of iron strip 70and aids in cooling and casting iron strip 70. For example, thetemperatures of pinch rollers 72 may be maintained between about 300 K(about 26.85° C.) and about 400 K (about 126.85° C.). The pressureexerted on iron ingot by pinch rollers 72 may be between about 0.04 MPaand about 0.1 MPa.

After iron strip 70 is pressed between pinch rollers 72 and cooled, ironstrip 70 forms textured iron sheets 74 a and 74 b. In some examples,textured iron sheets 74 a and 74 b (collectively, “textured iron sheets74”) may form textured iron ribbons with at least one dimension (e.g., athickness) between about 1 μm and about 10 mm, such as between about 5μm and about 1 mm (either individually or after compression of multipleiron workpieces). Each of textured iron sheets 74 may include, forexample, a (100) or (110) crystal texture. In other words, a majorsurface of each of textured iron sheets 74 may be parallel to the (100)or (110) surfaces of all or substantially all of the iron crystalswithin the respective one of textured iron sheets 74. By using atextured iron sheet 74 in which all or substantially all of the ironcrystals have substantially aligned crystal axes in the subsequentprocessing steps, anisotropy formed when forming the Fe₈N and Fe₁₆N₂phase domains may be substantially aligned among the crystals.

Following formation of textured iron sheets 74 (52), N+ ions may beimplanted in the each of textured iron sheets 74 using ion implantation(12). The N+ ions may be implanted in the textured iron sheet 74 aand/or 74 b using techniques and parameters similar to those describedwith reference to FIG. 1. For example, the implant energy used toimplant the N+ ions may be selected based at least in part on thedimensions (e.g., thickness) of the respective one of textured ironsheets 74 in which the N+ ions are being implanted while avoiding overlysignificant damage to the one of textured iron sheets 74, including thecrystal lattice of the iron crystals in the one of textured iron sheets74. For example, while higher implant energies may allow implantation ofthe N+ ions at a greater average depth, higher implant energies mayincrease the damage to textured iron sheets 74, including damaging thecrystal lattice of the iron crystals and ablating some of the iron atomsdue to the impact of the N+ ions. Hence, in some examples, the implantenergy may be limited to be below about 180 keV, such as about 100 keV.

In some examples, the incident angle of implantation may be about zerodegrees (e.g., substantially perpendicular to the surface of thetextured iron sheets 74). In other examples, the incident angle ofimplantation may be adjusted to reduce lattice damage. For example, theincident angle of implantation may be between about 3° and about 7° fromperpendicular.

The temperature of the textured iron sheets 74 during the ionimplantation also may be controlled. In some examples, the temperatureof the textured iron sheets 74 may be between about room temperature andabout 500° C.

Additionally, the fluency of N+ ions may be selected to implant adesired dose of N+ ions within the respective one of textured ironsheets 74. In some examples, the fluency of N+ ions may be selected toimplant approximately stoichiometric number of N+ ions within therespective one of textured iron sheets 74. The stoichiometric ratio ofiron to nitrogen in Fe₁₆N₂ is 8:1. Thus, the approximate number of ironatoms in the respective one of textured iron sheets 74 may bedetermined, and a number of N+ ions equal to approximately ⅛ (12.5%) ofthe iron atoms may be implanted in the one of textured iron sheets 74,such as between about 8 at. % and about 15 at. %.

Once the selected number of N+ ions has been implanted in the texturediron sheet 74 a or 74 b to form a textured iron nitride sheet, thetextured iron nitride sheet may be annealed to allow diffusion of N+ions into appropriate positions within the iron crystals to form Fe₁₆N₂phase domains and Fe₈N phase domains within the textured iron nitridesheet (16). The annealing at this step may be similar to thepost-annealing step described with respect to FIG. 1. As shown in FIG.3, annealing at relatively low temperatures allows transformation ofpartial Fe₈N disordered phase into Fe₁₆N₂ ordered phase. In someexamples, the post-annealing step may be carried out at a temperaturebelow about 250° C., such as between about 120° C. and about 214° C.,between about 120° C. and about 200° C., between about 150° C. and about200° C., or at about 150° C. The post-annealing step may be performed ina nitrogen (N₂) or argon (Ar) atmosphere, or in a vacuum or near-vacuum.

The temperature and duration of the post-annealing step may be selectedbased on, for example, a size of the textured iron nitride workpiece(e.g., the textured iron nitride sheet) and a diffusion coefficient ofnitrogen atoms in iron at the post-annealing temperature. Based on thesefactors, the temperature and duration may be selected to providesufficient time for nitrogen atoms to diffuse to locations within thetextured iron nitride workpiece to form Fe₁₆N₂ domains.

Additionally, the temperature and duration of the post-annealing stepmay be selected based on a desired volume fraction of Fe₁₆N₂ phasedomains in the textured iron nitride workpiece. For example, at aselected temperature, a longer post-annealing step may result in ahigher volume fraction of Fe₁₆N₂. Similarly, for a given post-annealingstep duration, a higher temperature may result in a higher volumefraction of Fe₁₆N₂. However, for durations above a threshold value, theadditional volume fraction of Fe₁₆N₂ may be limited or eliminated, asthe volume fraction of Fe₁₆N₂ reaches a relatively stable value. Theduration of the post-annealing step may be at least about 5 hours, suchas at least about 20 hours, or between about 5 hours and about 100hours, or between about 20 hours and about 100 hours, or about 40 hours.

During the post-annealing step, the textured iron nitride workpiece maybe subjected to a strain to facilitate transformation of at least someof the bcc iron crystals into a bct crystal structure. The stain may beexerted on the texture iron nitride workpiece using a variety of straininducing apparatuses. For example, a first end of the textured ironnitride workpiece may be received by (e.g., wound around) a first rollerand a second end of the textured iron nitride workpiece may be receivedby (e.g., wound around) a second roller. The rollers may be rotated inopposite directions to exert a tensile force on the textured ironnitride workpiece.

In other examples, opposite ends the textured iron nitride workpiece maybe gripped in mechanical grips, e.g., clamps, and the mechanical gripsmay be moved away from each other to exert a tensile force on thetextured iron nitride workpiece. FIG. 11 is a conceptual diagramillustrating another example apparatus with which the textured ironnitride workpiece can be strained as described herein. Apparatus 80 mayinclude first clamp 82 and second clamp 84, which may secure opposingends of a textured iron nitride workpiece 90 by tightening screws 86a-86 d. Once the iron workpiece 90 is secured in apparatus 80, bolt 88may be turned to rotate the threaded body of bolt 88 and increase thedistance between first clamp 82 and second clamp 84. The increase indistance between clamps 82 and 84 exerts a tensile force on texturediron nitride workpiece 90. The value of the elongation or stressgenerated by the rotation of bolt 88 may be measured by any suitablegauge, such as, e.g., a strain gauge. In some examples, apparatus 80 maybe placed in a furnace (e.g., a tube furnace) or other heatedenvironment to allow heating of textured iron nitride workpiece 90during and/or after textured iron nitride workpiece 90 is strained byapparatus 80.

A strain inducing apparatus may strain the textured iron nitrideworkpiece 90 to a certain elongation. For example, the strain on thetextured iron nitride workpiece 90 may be between about 0.1% and about7%. In other examples, the strain on the textured iron nitride sheet 90may be less than about 0.1% or greater than about 7%. In some examples,exerting a certain strain on the textured iron nitride workpiece 90 mayresult in a substantially similar strain on individual unit cells of theiron crystals, such that the unit cell is elongated along the <001> axisbetween about 0.1% and about 7%.

A cross-sectional area of the textured iron nitride workpiece (in aplane substantially orthogonal to the direction in which the ironworkpiece is stretched/strained) may affect an amount of force that mustbe applied to the textured iron nitride workpiece to result in a givenstrain. For example, the application of approximately 144 N of force toa textured iron nitride workpiece with a cross-sectional area of about0.00785 mm² may result in about a 7% strain. As another example, theapplication of approximately 576 N of force to a textured iron nitrideworkpiece with a cross-sectional area of about 0.0314 mm² may result inabout a 7% strain. As another example, the application of approximately1296 N of force to a textured iron nitride workpiece with across-sectional area of about 0.0707 mm² may result in about a 7%strain. As another example, the application of approximately 2304 N offorce to a textured iron nitride workpiece with a cross-sectional areaof about 0.126 mm² may result in about a 7% strain. As another example,the application of approximately 3600 N of force to a textured ironnitride workpiece with a cross-sectional area of about 0.196 mm² mayresult in about a 7% strain.

The post-annealing and straining step results in formation of Fe₁₆N₂phase domains within domains of Fe₈N, and other Fe and/or iron nitridecompositions, e.g., as shown in FIG. 6. Because the textured ironnitride workpiece is structured on a nanometer scale (e.g., the Fe₈Ndomains 22 and Fe₁₆N₂ domains 24 are on the order of nanometers),magnetic coupling between the magnetically hard Fe₁₆N₂ domains 24 andthe magnetically soft Fe₈N domains 22 may occur substantially throughoutthe workpiece. Because the Fe₁₆N₂ and Fe₈N crystals have substantiallysimilar crystalline structure, the material can be naturallycrystallographically coherent, meaning having an aligned easy axis,which produces anisotropy. This may facilitate exchange coupling throughphase boundaries between Fe₁₆N₂ and Fe₈N.

Together, the steps of forming the textured iron workpiece (e.g.,textured iron sheet) (52), implanting N+ ions using ion implantation(12) and annealing (16) in FIG. 9 may constitute forming an iron nitridematerial including Fe₁₆N₂ phase domains (32), shown in FIG. 7. Thetechnique of FIG. 9 also may include introducing additional iron ornonmagnetic materials between workpieces (e.g., sheets) of iron nitride(54), which is an example of introducing additional iron or nonmagneticmaterials (34), described with reference to FIG. 7. Example nonmagneticelements (e.g., atoms) or compounds (e.g., molecules that can be usedinclude Al, Cu, Ti, Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃, orcombinations thereof.

The iron or nonmagnetic materials may be introduced as workpieces ofmaterial or powders, and may be introduced between workpieces oftextured iron nitride including the Fe₁₆N₂ phase domains. The workpiecesof material or Fe or nonmagnetic powders may have sizes (e.g.,thicknesses or diameters) ranging from several nanometers to severalhundred nanometers, and may function as domain wall pinning sites aftersintering process. Those domain wall pinning sites may enhance thecoercivity of the permanent magnet.

The technique of FIG. 9 also may include sintering the workpieces (e.g.,sheets) to form a bulk magnetic material (36). As described with respectto FIGS. 7, 8A, and 8B, the sintering pressure, temperature and durationmay be selected to mechanically join the workpieces while maintainingthe crystal structure of the textured iron nitride workpieces, (e.g., asincluding the Fe₁₆N₂ phase domains). Thus, in some examples, thesintering step may be performed at a relatively low temperature. Forexample, the sintering temperature may be below about 250° C., such asbetween about 120° C. and about 250° C., between about 150° C. and about250° C., between about 120° C. and about 200° C., between about 150° C.and about 200° C., or about 150° C. The sintering pressure may bebetween, for example, about 0.2 GPa and about 10 GPa. The sintering timemay be at least about 5 hours, such as at least about 20 hours, orbetween about 5 hours and about 100 hours, or between about 20 hours andabout 100 hours, or about 40 hours. The sintering time, temperature, andpressure may be affected by the materials in workpieces 44 a, 44 b thatinclude iron or nonmagnetic materials.

After sintering together the textured iron nitride workpieces with theiron or nonmagnetic materials, a bulk magnetic material (e.g., a bulkpermanent magnet) may be formed. The bulk magnetic material may includeboth exchange spring coupling between Fe₈N and Fe₁₆N₂ and domain wallpinning sites provided by the iron or nonmagnetic materials. In thisway, the bulk magnetic material may possess a desirably high energyproduct, which may approach that of pure Fe₁₆N₂ (about 134 MGOe).

FIG. 12 is a flow diagram illustrating an example technique for forminga bulk material including Fe₁₆N₂ magnetically hard domainsexchange-spring coupled with Fe₈N magnetically soft domains, with domainwall pinning sites. The technique of FIG. 12 includes forming a texturediron nitride workpiece (e.g., a textured iron nitride sheet) (92). Thetextured iron nitride workpiece may be formed using, for example, beltcasting. FIG. 13 is a conceptual diagram illustrating one exampleapparatus 100 for fast belt casting to texture an example iron nitrideworkpiece.

Fast belt casting apparatus 10 may include an ingot chamber 66 whichcontains molten iron ingot 62, and is heated by heating source 64, e.g.,in the form of a heating coil. In some examples, the temperature ofmolten iron ingot 62 within ingot chamber 66 may be greater than about1800 Kelvin (K; about 1526.85° C.). The pressure of the iron ingot 62within ingot chamber 66 may be between about 0.06 MPa and about 0.12MPa.

Ingot chamber 66 also includes a nitrogen inlet 102, through which anitrogen source is introduced into molten iron ingot 62 to form a molteniron nitride mixture 104. Nitrogen may be provided through nitrogeninlet 102 in a variety of forms or from a variety of sources. Forexample, nitrogen may be provided in the form of ammonia, ammoniumazide, or urea, which may be introduced through nitrogen inlet 102, andthen break down to release nitrogen atoms upon mixing with molten ironin molten iron nitride mixture 104.

In some examples, the nitrogen source may be provided to result in anapproximately stoichiometric number of nitrogen atoms within the ironnitride mixture 104. The stoichiometric ratio of iron to nitrogen inFe₁₆N₂ is 8:1. Thus, the approximate number of iron atoms in ironnitride mixture 104 may be determined, and a number of nitrogen atomsequal to approximately ⅛ (12.5%) of the iron atoms may be providedthrough nitrogen inlet 102 to the iron nitride mixture 104, such asbetween about 8 at. % and about 15 at. %.

Molten iron nitride mixture 104 flows out of ingot chamber 66 throughnozzle head 68 to form iron nitride strip 106. Iron nitride strip 106 isfed into the gap zone between the surfaces of first pinch roller 72 aand second pinch roller 72 b (collectively, “pinch rollers 72”), whichare rotated in opposite directions. In some examples, the distance fromnozzle head 68 to the surfaces of pinch rollers 72 may be between about1 mm and about 50 mm, such as about 4 mm.

In some examples, the rotation speed of first pinch roller 72 a andsecond pinch roller 72 b may vary from approximately 10 rotations perminute (rpm) to 1000 rpm, and the rotation speed of the rollers 72 maybe approximately the same. In some examples, pinch rollers 72 areactively cooled, e.g., using water cooling, which maintains the surfacesof rollers 72 at a temperature below the temperature of iron nitridestrip 106 and aids in cooling and casting iron nitride strip 106. Forexample, the temperatures of pinch rollers 72 may be maintained betweenabout 300 K (about 26.85° C.) and about 400 K (about 126.85° C.). Thepressure exerted on the iron nitride strip 106 by pinch rollers 72 maybe between about 0.04 MPa and about 0.1 MPa.

After iron nitride strip 106 is pressed between pinch rollers 72 andcooled, iron nitride strip 106 forms textured iron nitride sheets 108 aand 108 b. In some examples, textured iron nitride sheets 108 a and 108b (collectively, “textured iron nitride sheets 108”) may form texturediron nitride ribbon with at least one dimension (e.g., a thickness)between about 1 μm and about 10 mm, such as between about 5 μm and about1 cm (either individually or after compression of multiple textured ironnitride sheets 108). Each of textured iron nitride sheets 108 mayinclude, for example, a (100) or (110) crystal texture. In other words,a major surface of each of textured iron nitride sheets 108 may beparallel to the (100) or (110) surfaces of all or substantially all ofthe iron crystals within the respective one of textured iron nitridesheets 108. By using a textured iron nitride sheet 108 a or 108 b inwhich all or substantially all of the iron crystals have substantiallyaligned crystal axes in the subsequent processing steps, anisotropyformed when forming the Fe₈N and Fe₁₆N₂ phase domains may besubstantially aligned among the crystals.

After forming textured iron nitride sheets 108 (92), the textured ironnitride sheets 108 may be annealed to form Fe₁₆N₂ phase domains and Fe₈Nphase domains (16). This step may be similar to or substantially thesame as described with respect to FIGS. 1 and 9. For example, thepost-annealing step may be carried out at a temperature below about 250°C., such as between about 120° C. and about 214° C., between about 120°C. and about 200° C., between about 150° C. and about 200° C., or atabout 150° C. The post-annealing step may be performed in a nitrogen(N₂) or argon (Ar) atmosphere, or in a vacuum or near-vacuum. Theduration of the post-annealing step may be at least about 5 hours, suchas at least about 20 hours, or between about 5 hours and about 100hours, or between about 20 hours and about 100 hours, or about 40 hours.

The temperature and duration of the post-annealing step may be selectedbased on, for example, a size of the textured iron nitride workpiece(e.g., textured iron nitride sheet), a diffusion coefficient of nitrogenatoms in iron at the post-annealing temperature, and a desired volumefraction of Fe₁₆N₂ phase domains in the textured iron nitride workpiece.Based on these factors, the temperature and duration may be selected toprovide sufficient time for nitrogen atoms to diffuse to locationswithin the textured iron nitride workpiece to form Fe₁₆N₂ domains.

During the post-annealing step, the textured iron nitride workpiece maybe subjected to a strain to facilitate transformation of at least someof the bcc iron crystals into a bct crystal structure. A strain inducingapparatus may strain the textured iron nitride workpiece to a certainelongation. For example, the strain on the textured iron nitrideworkpiece may be between about 0.1% and about 7%. In other examples, thestrain on the textured iron nitride workpiece may be less than about0.1% or greater than about 7%. In some examples, exerting a certainstrain on the textured iron nitride workpiece may result in asubstantially similar strain on individual unit cells of the ironcrystals, such that the unit cell is elongated along the <001> axisbetween about 0.1% and about 7%.

Together, the steps of forming the textured iron nitride workpiece(e.g., sheet) (92) and annealing the textured iron nitride workpiece(16) in FIG. 12 may constitute forming an iron nitride materialincluding Fe₁₆N₂ phase domains (32), shown in FIG. 7. The technique ofFIG. 12 also may include introducing additional iron or nonmagneticmaterials between workpieces (e.g., sheets) of iron nitride (54), whichis an example of introducing additional iron or nonmagnetic materials(34), described with reference to FIG. 7. Example nonmagnetic elements(e.g., atoms) or compounds (e.g., molecules) that can be used includeAl, Cu, Ti, Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃, or combinationsthereof.

The iron or nonmagnetic materials may be introduced as workpieces ofmaterial or powders, and may be introduced between workpieces of textureiron nitride including the Fe₁₆N₂ phase domains. The workpieces ormaterial or Fe or nonmagnetic powders may have sizes (e.g., thicknessesor diameters) ranging from several nanometers to several hundrednanometers, and may function as domain wall pinning sites aftersintering process. Those domain wall pinning sites may enhance thecoercivity of the permanent magnet.

The technique of FIG. 12 also may include sintering the workpieces(e.g., sheets) to form a bulk magnetic material (36). As described withrespect to FIGS. 7, 8A, and 8B, the sintering pressure, temperature andduration may be selected to mechanically join the workpieces whilemaintaining the crystal structure of the textured iron nitrideworkpieces, (e.g., as including the Fe₁₆N₂ phase domains). Thus, in someexamples, the sintering step may be performed at a relatively lowtemperature. For example, the sintering temperature may be below about250° C., such as between about 120° C. and about 250° C., between about150° C. and about 250° C., between about 120° C. and about 200° C.,between about 150° C. and about 200° C., or about 150° C. The sinteringpressure may be between, for example, about 0.2 GPa and about 10 GPa.The sintering time may be at least about 5 hours, such as at least about20 hours, or between about 5 hours and about 100 hours, or between about20 hours and about 100 hours, or about 40 hours. The sintering time,temperature, and pressure may be affected by the materials in workpieces44 a, 44 b that include iron or nonmagnetic materials.

After sintering together the textured iron nitride workpieces with theiron or nonmagnetic materials, a bulk magnetic material (e.g., a bulkpermanent magnet) may be formed. The bulk magnetic material may includeboth exchange spring coupling between Fe₈N and Fe₁₆N₂ and domain wallpinning sites provided by the iron or nonmagnetic materials. In thisway, the bulk magnetic material may possess a desirably high energyproduct, which may approach that of pure Fe₁₆N₂ (about 134 MGOe).

FIG. 14 is a flow diagram illustrating an example technique for forminga bulk material including Fe₁₆N₂ magnetically hard domainsexchange-spring coupled with Fe₈N magnetically soft domains, with domainwall pinning sites. The technique of FIG. 14 is similar to the techniquedescribed with respect to FIG. 9; however, instead of introducingadditional iron or nonmagnetic materials between workpieces (e.g.,sheets) of iron nitride (54), the technique of FIG. 14 includes dopingthe textured iron nitride workpieces including Fe₁₆N₂ phase domainsusing ion implantation or cluster implantation (108). Hence, thetechnique of FIG. 14 may include forming a textured iron workpiece (52),e.g., using belt casting. The technique of FIG. 14 also may includeimplanting N+ ions in the textured iron workpiece using ion implantation(12) to form a textured iron nitride workpiece and annealing thetextured iron nitride workpiece to form Fe₁₆N₂ phase domains (16).

The technique of FIG. 14 then includes doping the textured iron nitrideworkpieces including Fe₁₆N₂ phase domains using ion implantation orcluster implantation (108). Example nonmagnetic elements (e.g., atoms)or compounds (e.g., molecules) that can be used to dope the texturediron nitride workpieces include Al, Cu, Ti, Mn, Zr, Ta, B, C, Ni, Ru,SiO₂, Al₂O₃, or combinations thereof. By implanting these elements orcompounds within the textured iron nitride workpieces, the domain wallpinning effect may be increased compared to the examples in whichparticles or workpieces of these materials are interleaved with thetextured iron nitride workpieces before sintering the workpiecestogether. Increase domain wall pinning effects may improve magneticproperties of the final bulk magnetic material, including the coercivityof the bulk magnetic material.

The technique of FIG. 14 also includes sintering the doped, texturediron nitride workpieces to form the bulk magnetic material (36). Asdescribed with respect to FIGS. 7, 8A, and 8B, the sintering pressure,temperature and duration may be selected to mechanically join theworkpieces while maintaining the crystal structure of the textured ironnitride workpieces, (e.g., as including the Fe₁₆N₂ phase domains). Thus,in some examples, the sintering step may be performed at a relativelylow temperature. For example, the sintering temperature may be belowabout 250° C., such as between about 120° C. and about 250° C., betweenabout 150° C. and about 250° C., between about 120° C. and about 200°C., between about 150° C. and about 200° C., or about 150° C. Thesintering pressure may be between, for example, about 0.2 GPa and about10 GPa. The sintering time may be at least about 5 hours, such as atleast about 20 hours, or between about 5 hours and about 100 hours, orbetween about 20 hours and about 100 hours, or about 40 hours. Thesintering time, temperature, and pressure may be affected by thematerials in workpieces 44 a, 44 b that include iron or nonmagneticmaterials.

After sintering together the textured iron nitride workpieces with theiron or nonmagnetic materials, a bulk magnetic material (e.g., a bulkpermanent magnet) may be formed. The bulk magnetic material may includeboth exchange spring coupling between Fe₈N and Fe₁₆N₂ and domain wallpinning sites provided by the iron or nonmagnetic materials. In thisway, the bulk magnetic material may possess a desirably high energyproduct, which may approach that of pure Fe₁₆N₂ (about 134 MGOe).

FIG. 15 is a flow diagram illustrating an example technique for forminga bulk material including Fe₁₆N₂ magnetically hard domainsexchange-spring coupled with Fe₈N magnetically soft domains, with domainwall pinning sites. The technique of FIG. 15 will be described withconcurrent reference to FIGS. 16 and 17. FIG. 16 illustrates aconceptual diagram of an apparatus with which the iron workpiece can bestrained and exposed to nitrogen. FIG. 17 illustrates further detail ofone example of the cold crucible heating stage shown in FIG. 16.

The example apparatus of FIG. 16 includes a first roller 122, a secondroller 124, and a crucible heating stage 126. First roller 122 andsecond roller 124 are configured to receive a first end 138 and a secondend 140, respectively, of an iron workpiece 128. Iron workpiece 128defines a major axis between first end 138 and second end 140. As bestseen in FIG. 17, iron workpiece 128 passes through an aperture 130defined by crucible heating stage 126. Crucible heating stage 126includes an inductor 132 that surrounds at least a portion of theaperture 130 defined by crucible heating stage 126.

The example technique of FIG. 15 includes straining iron workpiece 128along a direction substantially parallel (e.g., parallel or nearlyparallel) to a <001> axis of at least one iron crystal in the ironworkpiece 128 (112). In some examples, iron workpiece 128 is formed ofiron having a body centered cubic (bcc) crystal structure.

In some examples, iron workpiece 128 is formed of a single bcc crystalstructure. In other examples, iron workpiece 128 may be formed of aplurality of bcc iron crystals. In some of these examples, the pluralityof iron crystals are oriented such that at least some, e.g., a majorityor substantially all, of the <001> axes of individual unit cells and/orcrystals are substantially parallel to the direction in which strain isapplied to iron workpiece 128. For example, when the iron is formed asiron workpiece 128, at least some of the <001> axes may be substantiallyparallel to the major axis of the iron workpiece 128, as shown in FIGS.16 and 17. As noted above, in some examples, single crystal iron nitrideworkpieces may be formed using crucible techniques. In addition to suchcrucible techniques, single crystal iron workpieces may be formed byeither the micro melt zone floating or pulling from a micro shaper toform iron workpiece 128.

In some examples, iron workpiece 128 may have a crystalline texturedstructure. Techniques that may be used to form crystalline textured(e.g., with desired crystalline orientation along the certain directionof workpieces) iron workpiece 128 include fast belt casting as describedwith reference to FIGS. 9 and 10.

The stain may be exerted on iron workpiece 128 using a variety of straininducing apparatuses. For example, as shown in FIG. 16, first end 138and second end 140 of iron workpiece 128 may be received by (e.g., woundaround) first roller 122 and second roller 124, respectively, androllers 122, 124 may be rotated in opposite directions (indicated byarrows 134 and 135 in FIG. 16) to exert a tensile force on the ironworkpiece 128. In other examples, opposite ends of iron workpiece 128may be gripped in mechanical grips, e.g., clamps, and the mechanicalgrips may be moved away from each other to exert a tensile force on theiron workpiece 128, as described above with reference to FIG. 11.

A strain inducing apparatus may strain iron workpiece 128 to a certainelongation. For example, the strain on iron workpiece 128 may be betweenabout 0.1% and about 7%. In other examples, the strain on iron workpiece128 may be less than about 0.1% or greater than about 7%. In someexamples, exerting a certain strain on iron workpiece 128 may result ina substantially similar strain on individual unit cells of the iron,such that the unit cell is elongated along the <001> axis between about0.1% and about 7%.

A dimension of the iron workpiece, such as, for example, a diameter ofthe iron wire or cross-sectional area of the iron sheet (in a planesubstantially orthogonal to the direction in which the iron sheet isstretched/strained) may affect an amount of force that must be appliedto iron workpiece 128 to result in a given strain. For example, theapplication of approximately 144 N of force to an iron wire with adiameter of about 0.1 mm may result in about a 7% strain. As anotherexample, the application of approximately 576 N of force to an iron wirewith a diameter of about 0.2 mm may result in about a 7% strain. Asanother example, the application of approximately 1296 N of force to aniron wire with a diameter of about 0.3 mm may result in about a 7%strain. As another example, the application of approximately 2304 N offorce to an iron wire with a diameter of about 0.4 mm may result inabout a 7% strain. As another example, the application of approximately3600 N of force to an iron wire with a diameter of about 0.5 mm mayresult in about a 7% strain.

In some examples, iron workpiece 128 may include dopant elements whichserve to stabilize the Fe₁₆N₂ phase constitution once the Fe₁₆N₂ phaseconstitution has been formed. For example, the phase stabilizationdopant elements may include cobalt (Co), titanium (Ti), copper (Cu),zinc (Zn), or the like.

As the strain inducing apparatus exerts the strain on iron workpiece 128and/or once the strain inducing apparatus is exerting a substantiallyconstant strain on the iron workpiece 128, iron workpiece 128 may benitridized (114). In some examples, during the nitridizing process, ironworkpiece 128 may be heated using a heating apparatus. One example of aheating apparatus that can be used to heat iron workpiece 128 iscrucible heating stage 126, shown in FIGS. 16 and 17.

Crucible heating stage 126 defines aperture 130 through which ironworkpiece 128 passes (e.g., in which a portion of iron workpiece 128 isdisposed). In some examples, no portion of crucible heating stage 126contacts iron workpiece 128 during the heating of iron workpiece 128. Insome implementations, this is advantageous as it lower a risk ofunwanted elements or chemical species contacting and diffusing into ironworkpiece 128. Unwanted elements or chemical species may affectproperties of iron workpiece 128; thus, it may be desirable to reduce orlimit contact between iron workpiece 128 and other materials.

Crucible heating stage 126 also includes an inductor 132 that surroundsat least a portion of aperture 130 defined by crucible heating stage126. Inductor 132 includes an electrically conductive material, such asaluminum, silver, or copper, through which an electric current may bepassed. The electric current may by an alternating current (AC), whichmay induce eddy currents in iron workpiece 128 and heat the ironworkpiece 128. In other examples, instead of using crucible heatingstage 126 to heat iron workpiece 128, other non-contact heating sourcesmay be used. For example, a radiation heat source, such as an infraredheat lamp, may be used to heat iron workpiece 128. As another example, aplasma arc lamp may be used to heat iron workpiece 128.

Regardless of the heating apparatus used to heat iron workpiece 128during the nitridizing process, the heating apparatus may heat ironworkpiece 128 to temperature for a time sufficient to allow diffusion ofnitrogen to a predetermined concentration substantially throughout thethickness, diameter, or internal volume of iron workpiece 128. In thismanner, the heating time and temperature are related, and may also beaffected by the composition and/or geometry of iron workpiece 128. Forexample, iron workpiece 128 may be heated to a temperature between about125° C. and about 700° C. for between about 2 hours and about 9 hours.In some examples, iron workpiece 128 may be heated to a temperaturebetween about 500° C. and about 660° C. for between about 2 hours andabout 4 hours.

In some examples, iron workpiece 128 includes an iron wire with adiameter of about 0.1 mm. In some of these examples, iron workpiece 128may be heated to a temperature of about 125° C. for about 8.85 hours ora temperature of about 600° C. for about 2.4 hours, or a temperature ofabout 660° C. for about 4 hours. In general, at a given temperature, thenitridizing process time may be inversely proportional to acharacteristic dimension squared of iron workpiece 128, such as adiameter of an iron wire or a thickness of an iron sheet.

In addition to heating iron workpiece 128, nitridizing iron workpiece128 (114) includes exposing iron workpiece 128 to an atomic nitrogensubstance, which diffuses into iron workpiece 128. In some examples, theatomic nitrogen substance may be supplied as diatomic nitrogen (N₂),which is then separated (cracked) into individual nitrogen atoms. Inother examples, the atomic nitrogen may be provided from another atomicnitrogen precursor, such as ammonia (NH₃). In other examples, the atomicnitrogen may be provided from urea (CO(NH₂)₂) or ammonium azide((NH₄)N₃).

The nitrogen may be supplied in a gas phase alone (e.g., substantiallypure ammonia, ammonium azide, or urea, or diatomic nitrogen gas) or as amixture with a carrier gas. In some examples, the carrier gas is argon(Ar). The gas or gas mixture may be provided at any suitable pressure,such as between about 0.001 Torr (about 0.133 pascals (Pa)) and about 10Torr (about 1333 Pa), such as between about 0.01 Torr (about 1.33 Pa)and about 0.1 Torr (about 13.33 Torr). In some examples, when thenitrogen is delivered as part of a mixture with a carrier gas, thepartial pressure of nitrogen or the nitrogen precursor (e.g., NH₃) maybe between about 0.02 and about 0.1.

The nitrogen precursor (e.g., N₂ or NH₃) may be cracked to form atomicnitrogen substances using a variety of techniques. For example, thenitrogen precursor may be heated using radiation to crack the nitrogenprecursor to form atomic nitrogen substances and/or promote reactionbetween the nitrogen precursor and iron workpiece 128. As anotherexample, a plasma arc lamp may be used to split the nitrogen precursorto form atomic nitrogen substances and/or promote reaction between thenitrogen precursor and iron workpiece 28.

In some examples, iron workpiece 128 may be nitridized (114) via a ureadiffusion process, in which urea is utilized as a nitrogen source (e.g.,rather than diatomic nitrogen or ammonia). Urea (also referred to ascarbamide) is an organic compound with the chemical formula CO(NH₂)₂that may be used in some cases as a nitrogen release fertilizer. Tonitridize iron workpiece 128 (114), urea may heated, e.g., within afurnace with iron workpiece 128, to generate decomposed nitrogen atomswhich may diffuse into iron workpiece 128. As will be described furtherbelow, the constitution of the resulting nitridized iron material may becontrolled to some extent by the temperature of the diffusion process aswell as the ratio (e.g., the weight ratio) of iron to urea used for theprocess. In other examples, iron workpiece 128 may be nitridized by animplantation process similar to that used in semiconductor processes forintroducing doping agents.

FIG. 18 is a schematic diagram illustrating an example apparatus 164that may be used for nitridizing iron workpiece 128 via a urea diffusionprocess. Such a urea diffusion process may be used to nitridizing ironworkpiece 128, e.g., when having a single crystal iron, a plurality ofcrystal structure, or textured structure. Moreover, iron materials withdifferent shapes, such as wire, sheet or bulk, can also be diffusedusing such a process. For wire material, the wire diameter may bevaried, e.g., from several micrometers to millimeters. For sheetmaterial, the sheet thickness may be from, e.g., several nanometers tomillimeters. For bulk material, the material weight may be from, e.g.,about 1 milligram to kilograms.

As shown, apparatus 164 includes crucible 166 within vacuum furnace 168.Iron workpiece 128 is located within crucible 166 along with thenitrogen source of urea 172. As shown in FIG. 18, a carrier gasincluding Ar and hydrogen is fed into crucible 166 during the ureadiffusion process. In other examples, a different carrier gas or even nocarrier gas may be used. In some examples, the gas flow rate withinvacuum furnace 168 during the urea diffusion process may be betweenapproximately 5 standard cubic centimeters per minute (sccm) toapproximately 50 sccm, such as, e.g., 20 sccm to approximately 50 sccmor 5 sccm to approximately 20 sccm.

Heating coils 170 may heat iron workpiece 128 and urea 172 during theurea diffusion process using any suitable technique, such as, e.g., eddycurrent, inductive current, radio frequency, and the like. Crucible 166may be configured to withstand the temperature used during the ureadiffusion process. In some examples, crucible 166 may be able towithstand temperatures up to approximately 1600° C.

Urea 172 may be heated with iron workpiece 128 to generate nitrogen thatmay diffuse into iron workpiece 128 to form an iron nitride material. Insome examples, urea 172 and iron workpiece 128 may heated toapproximately 650° C. or greater within crucible 166 followed by coolingto quench the iron and nitrogen mixture to form an iron nitride materialhaving a Fe₁₆N₂ phase constitution substantially throughout thethickness, diameter, or volume of iron workpiece 128. In some examples,urea 172 and iron workpiece 128 may heated to approximately 650° C. orgreater within crucible 166 for between approximately 5 minutes toapproximately 1 hour. In some examples, urea 172 and iron workpiece 128may be heated to between approximately 1000° C. to approximately 1500°C. for several minutes to approximately an hour. The time of heating maydepend on nitrogen thermal coefficient in different temperature. Forexample, if the iron workpiece defines a thickness is about 1micrometer, the diffusion process may be finished in about 5 minutes atabout 1200° C., about 12 minutes at 1100° C., and so forth.

To cool the heated material during the quenching process, cold water maybe circulated outside the crucible to rapidly cool the contents. In someexamples, the temperature may be decreased from 650° C. to roomtemperature in about 20 seconds.

As will be described below, in some examples, the temperature of urea172 and iron workpiece 128 may be between, e.g., approximately 120° C.and approximately 250° C. to anneal the iron and nitrogen mixture toform an iron nitride material having a Fe₁₆N₂ phase constitutionsubstantially throughout the thickness, diameter, or volume of ironworkpiece 128. Urea 172 and iron workpiece 128 may be at the annealingtemperature, e.g., for at least about 1 hour. Such an annealing processcould be used in addition to or as an alternative to other nitrogendiffusion techniques, e.g., when the iron material is single crystaliron workpiece, or textured iron workpiece with at least one dimensionin the micrometer level. In each of annealing and quenching, nitrogenmay diffuse into iron workpiece 128 from the nitrogen gas or gas mixtureincluding Ar plus hydrogen carrier gas within furnace 68. In someexamples, gas mixture may have a composition of approximately 86% Ar+4%H₂+10% N₂. In other examples, the gas mixture may have a composition of10% N₂+90% Ar or 100% N₂ or 100% Ar.

As will be described further below, the constitution of the iron nitridematerial formed via the urea diffusion process may be dependent on theweight ratio of urea to iron used. As such, in some examples, the weightratio of urea to iron may be selected to form an iron nitride materialhaving a Fe₁₆N₂ phase constitution. However, such a urea diffusionprocess may be used to form iron nitride materials other than thathaving a Fe₁₆N₂ phase constitution, such as, e.g., Fe₂N, Fe₃N, Fe₄N,Fe₈N, and the like.

Regardless of the technique used to nitridize iron workpiece 128 (14),the nitrogen may be diffused into iron workpiece 128 to a concentrationof between about 8 atomic percent (at. %) and about 14 at. %, such asabout 11 at. %. The concentration of nitrogen in iron may be an averageconcentration, and may vary throughout the volume of iron workpiece 128.In some examples, the resulting phase constitution of at least a portionof the nitridized iron workpiece 128 (after nitridizing iron workpiece128 (114)) may be α′ phase Fe₈N. The Fe₈N phase constitution is thechemically disordered counterpart of chemically-ordered Fe₁₆N₂ phase. AFe₈N phase constitution is also has a bct crystal structure, and canintroduce a relatively high magnetocrystalline anisotropy.

In some examples, once iron workpiece 128 has been nitridized (114), andwhile still being strained (112), iron workpiece 128 may be annealed ata temperature for a time to facilitate diffusion of the nitrogen atomsinto appropriate interstitial spaces within the iron lattice to formFe₁₆N₂ (16). The annealing process used in FIG. 15 may be similar to orsubstantially the same as that described with respect to FIGS. 1, 9, 12,and 14. For example, the post-annealing step may be carried out at atemperature below about 250° C., such as between about 120° C. and about214° C., between about 120° C. and about 200° C., between about 150° C.and about 200° C., or at about 150° C. The post-annealing step may beperformed in a nitrogen (N₂) or argon (Ar) atmosphere, or in a vacuum ornear-vacuum. The duration of the post-annealing step may be at leastabout 5 hours, such as at least about 20 hours, or between about 5 hoursand about 100 hours, or between about 20 hours and about 100 hours, orabout 40 hours.

The temperature and duration of the post-annealing step may be selectedbased on, for example, a size of the textured iron nitride workpiece, adiffusion coefficient of nitrogen atoms in iron at the post-annealingtemperature, and a desired volume fraction of Fe₁₆N₂ phase domains inthe textured iron nitride workpiece. Based on these factors, thetemperature and duration may be selected to provide sufficient time fornitrogen atoms to diffuse to locations within the textured iron nitrideworkpiece to form Fe₁₆N₂ domains.

Once the annealing process has been completed, iron workpiece 128 may becooled under vacuum or an inert atmosphere, such as argon, to reduce orprevent oxidation.

Although not illustrated in FIG. 15, in some examples, the technique mayinclude introducing additional iron or nonmagnetic materials betweenmultiple iron workpieces 128 (54) (FIGS. 9 and 12) or doping the ironworkpiece 128 using ion implantation or cluster implantation (108) (FIG.14).

In some examples, iron workpiece 128 may not be a sufficient size forthe desired application. In such examples, multiple iron workpieces 128may be formed (each including or consisting essentially of a Fe₁₆N₂phase constitution) and the multiple iron workpieces 128 may be sinteredtogether to form a larger permanent magnet that includes a Fe₁₆N₂ phaseconstitution (36). As described with respect to FIGS. 7, 8A, and 8B, thesintering pressure, temperature and duration may be selected tomechanically join the workpieces while maintaining the crystal structureof the textured iron nitride workpieces, (e.g., as including the Fe₁₆N₂phase domains). Thus, in some examples, the sintering step may beperformed at a relatively low temperature. For example, the sinteringtemperature may be below about 250° C., such as between about 120° C.and about 250° C., between about 150° C. and about 250° C., betweenabout 120° C. and about 200° C., between about 150° C. and about 200°C., or about 150° C. The sintering pressure may be between, for example,about 0.2 GPa and about 10 GPa. The sintering time may be at least about5 hours, such as at least about 20 hours, or between about 5 hours andabout 100 hours, or between about 20 hours and about 100 hours, or about40 hours. The sintering time, temperature, and pressure may be affectedby the materials in the workpieces or powder that include iron ornonmagnetic materials.

After sintering together the textured iron nitride workpieces with theiron or nonmagnetic materials, a bulk magnetic material (e.g., a bulkpermanent magnet) may be formed. The bulk magnetic material may includeboth exchange spring coupling between Fe₈N and Fe₁₆N₂ and domain wallpinning sites provided by the iron or nonmagnetic materials. In thisway, the bulk magnetic material may possess a desirably high energyproduct, which may approach that of pure Fe₁₆N₂ (about 134 MGOe).

Clause 1: A bulk permanent magnetic material comprising between about 5volume percent and about 40 volume percent Fe₁₆N₂ phase domains; aplurality of nonmagnetic elements or compounds forming domain wallpinning sites; and a balance soft magnetic material, wherein at leastsome of the soft magnetic material is magnetically coupled to the Fe₁₆N₂phase domains via exchange spring coupling.

Clause 2: The bulk permanent magnetic material of clause 1, comprisingbetween about 5 volume percent and about 20 volume percent Fe₁₆N₂ phasedomains.

Clause 3: The bulk permanent magnetic material of clause 1, comprisingbetween about 10 volume percent and about 15 volume percent Fe₁₆N₂ phasedomains.

Clause 4: The bulk permanent magnetic material of any of clauses 1 to 3,wherein the Fe₁₆N₂ phase domains are distributed throughout a volume ofthe bulk permanent magnetic material.

Clause 5: The bulk permanent magnetic material of any of clauses 1 to 4,wherein the plurality of nonmagnetic elements of compounds comprises anelement or compound selected from the group consisting of Al, Cu, Ti,Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃, or combinations thereof.

Clause 6: The bulk permanent magnetic material of any of clauses 1 to 5,wherein the soft magnetic material is selected from the group consistingof Fe₈N, Fe₄N, Fe, FeCo, and combinations thereof.

Clause 7: The bulk permanent magnetic material of any of clauses 1 to 5,wherein the soft magnetic material comprises Fe₈N.

Clause 8: The bulk permanent magnetic material of any of clauses 1 to 7,wherein the bulk permanent magnetic comprises a plurality of sinteredworkpieces of iron nitride, each workpiece of iron nitride includingbetween about 5 volume percent and about 40 volume percent Fe₁₆N₂ phasedomains.

Clause 9: The bulk permanent magnetic material of any of clauses 1 to 8,wherein a smallest dimension of the bulk permanent magnetic material isgreater than about 100 nanometers.

Clause 10: The bulk permanent magnetic material of clause 9, wherein thesmallest dimension is greater than about 1 micrometer.

Clause 11: The bulk permanent magnetic material of clause 10, whereinthe smallest dimension is greater than about 100 micrometers.

Clause 12: The bulk permanent magnetic material of any of clauses 1 to11, wherein the magnetic material has an energy product of greater thanabout 10 MGOe.

Clause 13: The bulk permanent magnetic material of clause 12, whereinthe magnetic material has an energy product of greater than about 30MGOe.

Clause 14: The bulk permanent magnetic material of clause 13, whereinthe magnetic material has an energy product of greater than about 60MGOe.

Clause 15: The bulk permanent magnetic material of clause 14, whereinthe magnetic material has an energy product of greater than about 100MGOe.

Clause 16: The bulk permanent magnetic material of clause 12, whereinthe magnetic material has an energy product of between about 60 MGOe andabout 135 MGOe.

Clause 17: The bulk permanent magnetic material of any of clauses 1 to16, wherein the material is naturally crystallographically coherent.

Clause 18: A method comprising implanting N+ ions in an iron workpieceusing ion implantation to form an iron nitride workpiece; pre-annealingthe iron nitride workpiece to attach the iron nitride workpiece to asubstrate; and post-annealing the iron nitride workpiece to form Fe₁₆N₂phase domains within the iron nitride workpiece.

Clause 19: The method of clause 18, wherein implanting N+ ions in theiron workpiece using ion implantation to form the iron nitride workpiececomprises accelerating N+ ions to an energy of less than about 180kiloelectron volts.

Clause 20: The method of clause 19, wherein accelerating N+ ions to theenergy of less than about 180 kiloelectronvolts comprises acceleratingN+ ions to the energy of about 100 kiloelectronvolts.

Clause 21: The method of any of clauses 18 to 20, wherein implanting N+ions in the iron workpiece using ion implantation to form the ironnitride workpiece comprises providing N+ ions at a fluence of betweenabout 2×10¹⁶/cm² and about 1×10¹⁷/cm².

Clause 22: The method of any of clauses 18 to 20, wherein implanting N+ions in the iron workpiece using ion implantation to form the ironnitride workpiece comprises providing N+ ions at a fluence of about8×10¹⁶/cm².

Clause 23: The method of any of clauses 18 to 22, wherein implanting N+ions in the iron workpiece using ion implantation to form the ironnitride workpiece comprises providing sufficient N+ ions to form anaverage concentration of nitrogen in the iron workpiece between about 8atomic percent and about 15 atomic percent.

Clause 24: The method of any of clauses 18 to 22, wherein implanting N+ions in the iron workpiece using ion implantation to form the ironnitride workpiece comprises providing sufficient N+ ions to form anaverage concentration of nitrogen in the iron workpiece of about 12.5atomic percent.

Clause 25: The method of any of clauses 18 to 24, wherein the ironworkpiece defines a thickness of between about 500 nanometers and about1 millimeter prior to implanting N+ ions in the iron workpiece using ionimplantation to form the iron nitride workpiece.

Clause 26: The method of clause 25, wherein the iron workpiece defines athickness of about 500 nanometers prior to implanting N+ ions in theiron workpiece using ion implantation to form the iron nitrideworkpiece.

Clause 27: The method of any of clauses 18 to 26, wherein the ironworkpiece comprises a plurality of iron crystals, and wherein a majorsurface of the iron workpiece is substantially parallel (110) surfacesof all or substantially all of the iron crystals.

Clause 28: The method of any of clauses 18 to 27, wherein the substratecomprises silicon or gallium arsenide.

Clause 29: The method of clause 28, wherein the substrate comprises asingle crystal silicon substrate with a (111) major surface.

Clause 30: The method of clause 29, wherein pre-annealing the ironnitride workpiece to attach the iron nitride workpiece to the substratecomprises pre-annealing the iron nitride workpiece to attach the ironnitride workpiece to the (111) major surface of the single crystalsilicon substrate.

Clause 31: The method of any of clauses 18 to 30, wherein pre-annealingthe iron nitride workpiece to attach the iron nitride workpiece to thesubstrate comprises applying an external force between about 0.2gigapascals and about 10 gigapascals between the iron workpiece and thesubstrate.

Clause 32: The method of any of clauses 18 to 31, wherein pre-annealingthe iron nitride workpiece to attach the iron nitride workpiece to thesubstrate comprises heating the iron nitride workpiece and the substrateto a temperature between about 450° C. and about 550° C. for betweenabout 30 minutes and about 4 hours.

Clause 33: The method of clause 32, wherein pre-annealing the ironnitride workpiece to attach the iron nitride workpiece to the substratecomprises heating the iron nitride workpiece and the substrate to atemperature of about 500° C. for between about 30 minutes and about 1hour.

Clause 34: The method of clause 32 or 33, wherein pre-annealing the ironnitride workpiece to attach the iron nitride workpiece to the substratecomprises pre-annealing the iron nitride workpiece under an atmospherecomprising nitrogen and argon.

Clause 35: The method of clause 34, wherein pre-annealing the ironnitride workpiece to attach the iron nitride workpiece to the substratecomprises pre-annealing the iron nitride workpiece under an atmospherecomprising about 10 volume percent nitrogen, about 86 volume percentargon and about 4 volume percent hydrogen.

Clause 36: The method of any of clauses 18 to 35, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises heating the iron nitride workpiece and thesubstrate to a temperature between about 120° C. and about 250° C. forat least about 5 hours.

Clause 37: The method of any of clauses 18 to 35, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises heating the iron nitride workpiece and thesubstrate to a temperature between about 120° C. and about 200° C. forbetween about 20 hours and about 100 hours.

Clause 38: The method of any of clauses 18 to 35, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises heating the iron nitride workpiece and thesubstrate to a temperature of about 150° C. for between about 20 hoursand about 40 hours.

Clause 39: The method of any of clauses 18 to 38, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises forming between about 5 volume percent andabout 40 volume percent of Fe₁₆N₂ phase domains within the iron nitrideworkpiece.

Clause 40: The method of any of clauses 18 to 38, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises forming between about 10 volume percent andabout 20 volume percent of Fe₁₆N₂ phase domains within the iron nitrideworkpiece.

Clause 41: The method of any of clauses 18 to 38, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises forming between about 10 volume percent andabout 15 volume percent of Fe₁₆N₂ phase domains within the iron nitrideworkpiece.

Clause 42: The method of any of clauses 18 to 41, wherein post-annealingthe iron nitride workpiece to form Fe₁₆N₂ phase domains within the ironnitride workpiece comprises forming Fe₁₆N₂ distributed throughout avolume of the iron nitride workpiece.

Clause 43: A method comprising forming a plurality of workpieces of ironnitride material, each of the plurality of workpieces including betweenabout 5 volume percent and about 40 volume percent of Fe₁₆N₂ phasedomains; introducing additional iron or nonmagnetic material between theplurality of workpieces or within at least one of the plurality ofworkpieces of iron nitride material; and sintering together theplurality of workpieces of iron nitride to form a bulk magnet includingiron nitride with between about 5 volume percent and about 40 volumepercent of Fe₁₆N₂ phase domains.

Clause 44: The method of clause 43, wherein forming the plurality ofworkpieces of iron nitride material comprises implanting N+ ions in atextured iron workpiece using ion implantation to form a textured ironnitride workpiece; and post-annealing the textured iron nitrideworkpiece to form Fe₁₆N₂ phase domains within the textured iron nitrideworkpiece.

Clause 45: The method of clause 44, further comprising forming thetextured iron workpiece using fast belt casting.

Clause 46: The method of clause 44 or 45, wherein the textured ironworkpiece defines a thickness between about 1 micrometer and about 10millimeters.

Clause 47: The method of clause 44 or 45, wherein the textured ironworkpiece defines a thickness between about 5 micrometer and about 1millimeter.

Clause 48: The method of any of clauses 44 to 47, wherein the texturediron workpiece includes a (100) or a (110) crystal structure.

Clause 49: The method of any of clauses 44 to 48, wherein implanting N+ions in the textured workpiece using ion implantation to form thetextured iron nitride workpiece comprises accelerating N+ ions to anenergy of less than about 180 kiloelectron volts.

Clause 50: The method of any of clauses 44 to 48, wherein acceleratingN+ ions to the energy of less than about 180 kiloelectronvolts comprisesaccelerating N+ ions to the energy of about 100 kiloelectronvolts.

Clause 51: The method of any of clauses 44 to 50, wherein implanting N+ions in the textured workpiece using ion implantation to form thetextured iron nitride workpiece comprises providing N+ ions at a fluenceof between about 2-10¹⁶/cm² and about 1×10¹⁷/cm².

Clause 52: The method of any of clauses 44 to 50, wherein implanting N+ions in the iron workpiece using ion implantation to form the ironnitride workpiece comprises providing N+ ions at a fluence of between ofabout 8×10¹⁶/cm².

Clause 53: The method of any of clauses 44 to 52, wherein implanting N+ions in the textured workpiece using ion implantation to form thetextured iron nitride workpiece comprises providing sufficient N+ ionsto form an average concentration of nitrogen in the textured ironnitride workpiece between about 8 atomic percent and about 15 atomicpercent.

Clause 54: The method of any of clauses 44 to 52, wherein implanting N+ions in the textured workpiece using ion implantation to form thetextured iron nitride workpiece comprises providing sufficient N+ ionsto form an average concentration of nitrogen in the textured ironnitride workpiece of about 12.5 atomic percent.

Clause 55: The method of clause 43, wherein forming the plurality ofworkpieces of iron nitride material comprises mixing a nitrogen sourcein molten iron; fast belt casting the molten iron to form a texturediron nitride workpiece; and post-annealing the textured iron nitrideworkpiece to form Fe₁₆N₂ phase domains within the textured iron nitrideworkpiece.

Clause 56: The method of clause 55, wherein mixing nitrogen in molteniron comprises mixing the nitrogen source in molten iron to result in aconcentration of nitrogen atoms in the molten iron between about 8atomic percent and about 15 atomic percent.

Clause 57: The method of clause 55, wherein mixing nitrogen in molteniron comprises mixing the nitrogen source in molten iron to result in aconcentration of nitrogen atoms in the molten iron of about 12.5 atomicpercent.

Clause 58: The method of any of clauses 55 to 57, wherein the nitrogensource comprises at least one of ammonia, ammonium azide, or urea.

Clause 59: The method of any of clauses 55 to 58, wherein the texturediron nitride workpiece includes a (100) or a (110) crystal structure.

Clause 60: The method of any of clauses 55 to 59, wherein the texturediron nitride workpiece defines a dimension between about 1 micrometerand about 10 millimeters.

Clause 61: The method of any of clauses 55 to 59, wherein the texturediron nitride workpiece defines a thickness between about 5 micrometerand about 1 millimeter.

Clause 62: The method of any of clauses 43 to 61, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises exerting a strain on thetextured iron nitride workpiece between about 0.1% and about 7%; andwhile exerting the strain on the textured iron nitride workpiece,heating the textured iron nitride workpiece to a temperature betweenabout 120° C. and about 250° C. for at least about 5 hours.

Clause 63: The method of clause 62, wherein heating the textured ironnitride workpiece to a temperature between about 120° C. and about 250°C. for at least about 5 hours comprises heating the textured ironnitride workpiece to a temperature between about 120° C. and about 200°C. for between about 20 hours and about 100 hours.

Clause 64: The method of clause 62, wherein heating the textured ironnitride workpiece to a temperature between about 120° C. and about 250°C. for at least about 5 hours comprises heating the textured ironnitride workpiece to a temperature of about 150° C. for between about 20hours and about 40 hours.

Clause 65: The method of any of clauses 44 to 64, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises forming between about 5volume percent and about 40 volume percent of Fe₁₆N₂ phase domainswithin the textured iron nitride workpiece.

Clause 66: The method of any of clauses 44 to 64, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises forming between about 5volume percent and about 20 volume percent of Fe₁₆N₂ phase domainswithin the textured iron nitride workpiece.

Clause 67: The method of any of clauses 44 to 64, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises forming between about 10volume percent and about 15 volume percent of Fe₁₆N₂ phase domainswithin the textured iron nitride workpiece.

Clause 68: The method of any of clauses 44 to 67, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises forming Fe₁₆N₂ distributedthroughout a volume of the textured iron nitride workpiece.

Clause 69: The method of any of clauses 43 to 68, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises implanting nonmagnetic ions within at least one ofthe plurality of workpieces of iron nitride material using ionimplantation.

Clause 70: The method of any of clauses 43 to 69, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises implanting nonmagnetic compounds within at least oneof the plurality of workpieces of iron nitride material using clusterimplantation.

Clause 71: The method of any of clauses 43 to 70, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises introducing workpieces of iron or nonmagneticmaterial between a first workpiece and a second workpiece of theplurality of workpieces of iron nitride material.

Clause 72: The method of any of clauses 43 to 71, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises introducing a powder comprising at least one of ironor nonmagnetic material between a first workpiece and a second workpieceof the plurality of workpieces of iron nitride material.

Clause 73: The method of any of clauses 43 to 71, wherein the iron ornonmagnetic material is selected from the group consisting of Al, Cu,Ti, Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃, or combinations thereof.

Clause 74: The method of any of clauses 43 to 73, wherein sinteringtogether the plurality of workpieces of iron nitride to form a bulkmagnet including iron nitride with between about 5 volume percent andabout 40 volume percent of Fe₁₆N₂ phase domains comprises heating theplurality of workpieces including iron nitride material to a temperaturebelow about 250° C. for at least about 5 hours.

Clause 75: The method of any of clauses 43 to 73, wherein sinteringtogether the plurality of workpieces of iron nitride to form a bulkmagnet including iron nitride with between about 5 volume percent andabout 40 volume percent of Fe₁₆N₂ phase domains comprises heating theplurality of workpieces of iron nitride to a temperature below about250° C. for at least about 5 hours while applying a pressure of betweenabout 0.2 gigapascal and about 10 gigapascals to the plurality ofworkpieces of iron nitride.

Clause 76: The method of any of clauses 43 to 73, wherein sinteringtogether the plurality of workpieces of iron nitride to form a bulkmagnet including iron nitride with between about 5 volume percent andabout 40 volume percent of Fe₁₆N₂ phase domains comprises heating theplurality of workpieces of iron nitride to a temperature between about120° C. and about 200° C. for between about 20 hours and about 100 hourswhile applying a pressure of between about 0.2 gigapascal and about 10gigapascals to the plurality of workpieces of iron nitride.

Clause 77: A method comprising forming a plurality of textured ironnitride workpieces by implanting N+ ions in a textured iron workpieceusing ion implantation to form a textured iron nitride workpiececomprising between about 8 atomic percent and about 15 atomic percent N+ions, and post-annealing the textured iron nitride workpiece to form avolume fraction of between about 5 volume percent and about 40 volumepercent of Fe₁₆N₂ phase domains within the textured iron nitrideworkpiece, with a balance soft magnetic material including Fe₈N, whereinat least some of the Fe₁₆N₂ phase domains are magnetically coupled to atleast one of the Fe₈N domains by exchange spring coupling; introducingnonmagnetic material between a first workpiece of the plurality ofworkpieces and a second workpiece of the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitride; andsintering together the plurality of workpieces of iron nitride to form abulk magnet including iron nitride with between about 5 volume percentand about 40 volume percent of Fe₁₆N₂ phase domains, wherein thenonmagnetic material forms domain wall pinning sites within the bulkmagnet.

Clause 78: The method of clause 77, further comprising forming thetextured iron workpiece using fast belt casting.

Clause 79: The method of clause 77 or 78, wherein the textured ironworkpiece defines a thickness between about 1 micrometer and about 10millimeters.

Clause 80: The method of clause 77 or 78, wherein the textured ironworkpiece includes a (100) or a (110) crystal structure.

Clause 81: The method of any of clauses 77 to 80, wherein implanting N+ions in the textured workpiece using ion implantation to form thetexture iron nitride sheet comprises accelerating N+ ions to the energyof about 100 kiloelectronvolts.

Clause 82: The method of any of clauses 77 to 81, wherein implanting N+ions in the iron workpiece using ion implantation to form the ironnitride workpiece comprises providing N+ ions at a fluence of between ofabout 8×10¹⁶/cm².

Clause 83: The method of any of clauses 77 to 82, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises exerting a strain on thetextured iron nitride workpiece between about 0.1% and about 7%; andwhile exerting the strain on the textured iron nitride workpiece,heating the textured iron nitride workpiece to a temperature betweenabout 120° C. and about 250° C. for at least about 5 hours.

Clause 84: The method of clause 83, wherein heating the textured ironnitride workpiece to a temperature between about 120° C. and about 250°C. for at least about 5 hours comprises heating the textured ironnitride workpiece to a temperature between about 120° C. and about 200°C. for between about 20 hours and about 100 hours.

Clause 85: The method of any of clauses 77 to 84, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises forming Fe₁₆N₂ distributedthroughout a volume of the textured iron nitride workpiece.

Clause 86: The method of any of clauses 77 to 85, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises implanting nonmagnetic ions within at least one ofthe plurality of workpieces of iron nitride material using ionimplantation.

Clause 87: The method of any of clauses 77 to 86, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises implanting nonmagnetic compounds within at least oneof the plurality of workpieces of iron nitride material using clusterimplantation.

Clause 88: The method of any of clauses 77 to 87, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises introducing workpieces of iron or nonmagneticmaterial between a first workpiece and a second workpiece of theplurality of workpieces of iron nitride material.

Clause 89: The method of any of clauses 77 to 88, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises introducing a powder comprising at least one of ironor nonmagnetic material between a first workpiece and a second workpieceof the plurality of workpieces of iron nitride material.

Clause 90: The method of any of clauses 77 to 89, wherein the iron ornonmagnetic material is selected from the group consisting of Al, Cu,Ti, Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃, or combinations thereof.

Clause 91: The method of any of clauses 77 to 90, wherein sinteringtogether the plurality of workpieces of iron nitride to form a bulkmagnet including iron nitride with between about 5 volume percent andabout 20 volume percent of Fe₁₆N₂ phase domains comprises heating theplurality of workpieces of iron nitride to a temperature between about120° C. and about 200° C. for between about 20 hours and about 100 hourswhile applying a pressure of between about 0.2 gigapascal and about 10gigapascals to the plurality of workpieces of iron nitride.

Clause 92: A method comprising forming a plurality of textured ironnitride workpieces by: mixing a nitrogen source in molten iron to resultin a concentration of nitrogen atoms in the molten iron between about 8atomic percent and about 15 atomic percent, fast belt casting the molteniron to form a textured iron nitride workpiece, and post-annealing thetextured iron nitride workpiece to form a volume fraction of betweenabout 5 volume percent and about 40 volume percent of Fe₁₆N₂ phasedomains within the textured iron nitride workpiece, with a balance softmagnetic material including Fe₈N, wherein at least some of the Fe₁₆N₂phase domains are magnetically coupled to at least one of the Fe₈Ndomains by exchange spring coupling; introducing nonmagnetic materialbetween a first workpiece of the plurality of workpieces and a secondworkpiece of the plurality of workpieces or within at least one of theplurality of workpieces of iron nitride, and sintering together theplurality of workpieces of iron nitride to form a bulk magnet includingiron nitride with between about 5 volume percent and about 40 volumepercent of Fe₁₆N₂ phase domains, wherein the nonmagnetic material formsdomain wall pinning sites within the bulk magnet.

Clause 93: The method of clause 92, wherein the nitrogen sourcecomprises at least one of ammonia, ammonium azide, or urea.

Clause 94: The method clause 92 or 93, wherein the textured iron nitrideworkpiece includes a (100) or a (110) crystal structure.

Clause 95: The method of any of clauses 92 to 94, wherein the texturediron nitride workpiece defines a thickness between about 1 micrometerand about 10 millimeters.

Clause 96: The method of any of clauses 92 to 95, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises exerting a strain on thetextured iron nitride workpiece between about 0.1% and about 7% andwhile exerting the strain on the textured iron nitride workpiece,heating the textured iron nitride workpiece to a temperature betweenabout 120° C. and about 250° C. for at least about 5 hours.

Clause 97: The method of clause 96, wherein heating the textured ironnitride workpiece to a temperature between about 120° C. and about 250°C. for at least about 5 hours comprises heating the textured ironnitride workpiece to a temperature between about 120° C. and about 200°C. for between about 20 hours and about 100 hours.

Clause 98: The method of any of clauses 92 to 97, wherein post-annealingthe textured iron nitride workpiece to form Fe₁₆N₂ phase domains withinthe textured iron nitride workpiece comprises forming Fe₁₆N₂ distributedthroughout a volume of the textured iron nitride workpiece.

Clause 99: The method of any of clauses 92 to 98, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises implanting nonmagnetic ions within at least one ofthe plurality of workpieces of iron nitride material using ionimplantation.

Clause 100: The method of any of clauses 92 to 99, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises implanting nonmagnetic compounds within at least oneof the plurality of workpieces of iron nitride material using clusterimplantation.

Clause 101: The method of any of clauses 92 to 100, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises introducing workpieces of iron or nonmagneticmaterial between a first workpiece and a second workpiece of theplurality of workpieces of iron nitride material.

Clause 102: The method of any of clauses 92 to 101, wherein introducingadditional iron or nonmagnetic between the plurality of workpieces orwithin at least one of the plurality of workpieces of iron nitridematerial comprises introducing a powder comprising at least one of ironor nonmagnetic material between a first workpiece and a second workpieceof the plurality of workpieces of iron nitride material.

Clause 103: The method of any of clauses 92 to 102, wherein the iron ornonmagnetic material is selected from the group consisting of Al, Cu,Ti, Mn, Zr, Ta, B, C, Ni, Ru, SiO₂, Al₂O₃, or combinations thereof.

Clause 104: The method of any of clauses 92 to 103, wherein sinteringtogether the plurality of workpieces of iron nitride to form a bulkmagnet including iron nitride with between about 5 volume percent andabout 20 volume percent of Fe₁₆N₂ phase domains comprises heating theplurality of workpieces of iron nitride to a temperature between about120° C. and about 200° C. for between about 20 hours and about 100 hourswhile applying a pressure of between about 0.2 gigapascal and about 10gigapascals to the plurality of workpieces of iron nitride.

Clause 105: A system for performing any of the techniques describedherein for forming a magnetic material including Fe₁₆N₂ phase domains.

EXAMPLES Example 1

FIG. 19 is a line diagram representing an auger measurement of N+ ionconcentration as a function of depth in an iron foil after ionimplantation and before annealing the iron nitride foil. Prior to N+ ionimplantation, the iron foil had a thickness of about 500 nm. N+ ionswere accelerated to 100 keV for implantation in the iron film. An N+ ionfluence of about 8×10¹⁶/cm² N+ ions was used to implant N+ ions in theiron foil. The measurement was performed using auger electronspectroscopy (AES) with Ar⁺ at the milling source using a PhysicalElectronics Industries (PHI) 545 scanning Auger microprobe, availablefrom Physical Electronics, Inc., Chanhassen, Minn. The peak position ofN+ ion concentration was about 1000 Å or 100 nm, as predicted by therelationship shown in FIG. 2. Additionally, N+ ions were implanted atother depths surrounding 1000 Å.

Example 2

FIG. 20 is a scatter diagram illustrating nitrogen concentrations as afunction of depth within an iron foil after post-annealing for differentnitrogen fluencies. Prior to N+ ion implantation, the iron foil had athickness of about 500 nm. N+ ions were accelerated to 100 keV forimplantation in the iron film. N+ ion fluences of about 2×10¹⁶/cm²,5×10¹⁶/cm², 8×10¹⁶/cm², and 1×10¹⁷/cm² N+ ions were used to implant N+ions in the iron foil. After ion implantation, the iron nitride foil wasattached to a (111) silicon substrate and subjected to a pre-annealingtreatment at about 500° C. for about 0.5 hours in a 4% H₂+10% N₂+86% Aratmosphere. The iron nitride foil was then subjected to a post-annealingtreatment at about 150° C. for about 40 hours in a vacuum.

As shown in FIG. 20, the thickness of the iron nitride foil after thepost-annealing step was about 450 nm. The loss of 50 nm of thickness isbelieved to be due to iron losses during ion bombardment and cleaningduring the pre-annealing step. As shown in FIG. 20, for each of thenitrogen fluencies, after the post-annealing step, the concentration ofnitrogen within the foil was substantially the same throughout the foil.The concentration of nitrogen for the 8 k 10¹⁶/cm² N+ ion fluency(upward pointing triangles) ranges from about 12.5 at. % to about 10 at.%, near the stoichiometric ratio of Fe:N in Fe₁₆N₂.

Example 3

FIGS. 21A and 21B are hysteresis loops of magnetization versuscoercivity for examples of an iron nitride foil prepared using ionimplantation. Prior to N+ ion implantation, the iron foil had athickness of about 500 nm. N+ ions were accelerated to 100 keV forimplantation in the iron film. N+ ion fluence of about 8×10¹⁶/cm² N+ions were used to implant N+ ions in the iron foil. After ionimplantation, the iron nitride foil was attached to a (111) siliconsubstrate and subjected to a pre-annealing treatment at about 500° C.for about 0.5 hours in a 4% H₂+10% N₂+86% Ar atmosphere. The ironnitride foil was then subjected to a post-annealing treatment at about150° C. for about 40 hours in a vacuum.

FIG. 21A illustrates the hysteresis loop of the iron nitride foil at atemperature of about 5 K. As shown in FIG. 21A, the pre- andpost-annealing treatments resulted in the saturation magnetizationincreasing from about 2.0 Tesla (T) to about 2.45 T. The pre- andpost-annealing treatments also increased the coercivity (H_(s)) fromabout 0 to about 3.2 kOe.

FIG. 21B illustrates the hysteresis loop of the iron nitride foil at atemperature of about 300 K. As shown in FIG. 21B, the pre- andpost-annealing treatments resulted in the saturation magnetizationincreasing from about 2.0 T to about 2.12 T. The pre- and post-annealingtreatments also increased the coercivity (H_(s)) from about 0 to about26 Oe. The difference in behavior between the foil tested at 5 K and thefoil tested at about 300 K is believed to be due to defects within thematerial that affect the results at the higher temperature, but whoseeffects are reduced at the lower temperature. Hence, the results at 5 Kare believed to be representative of ideal properties of some ironnitride films prepared using this technique.

Example 4

Pure (110) iron foils with a thickness of about 500 nm were positionedon mirror-polished (111) Si. Nitrogen ions of N+ were accelerated to 100keV and implanted into these foils vertically and at room temperaturewith doses of 2×10¹⁶/cm², 5×10¹⁶/cm², 8×10¹⁶/cm², and 1×10⁷/cm². Afterion implantation, a two-step annealing process was applied on theimplanted foils. The first step was pre-annealing at 500° C. in anatmosphere of nitrogen and argon for about 0.5 hour. After thepre-annealing step, a post-annealing treatment was performed at about150° C. for about 40 hours in vacuum.

The samples were exposed to the same implant energy but differentnitrogen fluencies. FIG. 22 is a line diagram that illustrates examplesof nitrogen depth profiles in iron foils before annealing. The nitrogendepth profiles were measured by Auger Electron Spectroscopy (AES) withAr+ as the in-depth milling source.

The nitrogen implant range inside the foil was determined by the implantenergy. As shown in FIG. 22, the four samples with different nitrogenfluencies have the same implant range (about 160 nm) and same peakposition (about 100 nm). This is coincident with the simulation resultby SRIM.

Example 5

Pure (110) iron foils with a thickness of about 500 nm were positionedon mirror-polished (111) Si. Nitrogen ions of N+ were accelerated to 100keV and implanted into these foils vertically and at room temperaturewith doses of 2×10¹⁶/cm², 5×10¹⁶/cm², 8×10¹⁶/cm², and 1×10¹⁷/cm². Afterion implantation, a two-step annealing process was applied on theimplanted foils. The first step was pre-annealing at 500° C. in anatmosphere of nitrogen and argon for about 0.5 hour. After thepre-annealing step, a post-annealing treatment was performed at about150° C. for about 40 hours in vacuum.

FIG. 23 is a scatter diagram illustrating nitrogen concentrations as afunction of depth within an iron foil after post-annealing for differentnitrogen fluencies. FIG. 23 shows that nitrogen concentrations in theannealed foils are substantially homogeneous for each nitrogen fluency.The nitrogen concentration distribution for the iron nitridecorresponding to 8×10¹⁶/cm² has reached to 11 at. %, close to thestoichiometric ratio of nitrogen in Fe₁₆N₂.

Example 6

Pure (110) iron foils with a thickness of about 500 nm were positionedon mirror-polished (111) Si. Nitrogen ions of N+ were accelerated to 100keV and implanted into these foils vertically and at room temperaturewith doses of 2×10¹⁶/cm², 5×10¹⁶/cm², 8×10¹⁶/cm², and 1×10¹⁷/cm². Afterion implantation, a two-step annealing process was applied on theimplanted foils. The first step was pre-annealing at 500° C. in anatmosphere of nitrogen and argon for about 0.5 hour. After thepre-annealing step, a post-annealing treatment was performed at about150° C. for about 40 hours in vacuum.

The crystal structure of foil samples was characterized using a SiemensD5005 X-ray diffractometer (XRD) with Cu Ka radiation source. FIG. 24illustrates XRD patterns collected before and after annealing for thefoil sample formed using an 8×10¹⁶/cm² fluence of nitrogen ions. For thespectrum before annealing, besides the silicon substrate Si (111) peak,a bcc Fe (110) peak appears. After annealing, the emerging Fe₁₆N₂ (330)(not labeled in FIG. 24) and (220) peaks indicate that part of bcc Fehas been transformed to Fe₁₆N₂ phase. FIG. 24 also illustrates adecrease in the intensity of the Fe₈N (330) peak after annealing.

Example 7

Pure (110) iron foils with a thickness of about 500 nm were positionedon mirror-polished (111) Si. Nitrogen ions of N+ were accelerated to 100keV and implanted into these foils vertically and at room temperaturewith doses of 8×10¹⁶/cm². After ion implantation, a two-step annealingprocess was applied on the implanted foils. The first step waspre-annealing at 500° C. in an atmosphere of nitrogen and argon forabout 0.5 hour. After the pre-annealing step, a post-annealing treatmentwas performed at about 150° C. for about 40 hours in vacuum.

FIG. 25 is a hysteresis loop of magnetization versus coercivity forexamples of an iron nitride foil prepared using ion implantation. Priorto ion implantation (curve 182 in FIG. 25), the magnetic properties ofthe iron foil were as expected for an iron single crystal, with remanentmagnetization being almost equal to saturation magnetization (about 2.02T at room temperature). This indicates an easy axis lying along thein-plane direction. After ion implantation and a pre-annealing step,saturation magnetization increased by about 7% compared to the ironfoil. After post-annealing, the saturation magnetization increased byabout 15% compared to the iron foil. After ion implantation,pre-annealing, and post-annealing, the saturation field (H_(s))increased to about 5.3 kOe, indicating a perpendicular magneticanisotropy due to the bct structure in Fe₁₆N₂.

Example 8

Pure (110) iron foils with a thickness of about 500 nm were positionedon mirror-polished (111) Si. Nitrogen ions of N+ were accelerated to 100keV and implanted into these foils vertically and at room temperaturewith doses of 2×10¹⁶/cm², 5×10¹⁶/cm², 8×10¹⁶/cm², and 1×10¹⁷/cm². Afterion implantation, a two-step annealing process was applied on theimplanted foils. The first step was pre-annealing at 500° C. in anatmosphere of nitrogen and argon for about 0.5 hour. After thepre-annealing step, a post-annealing treatment was performed at about150° C. for about 40 hours in vacuum.

To determine the chemical state for nitrogen in FeN foils before andafter annealing, X-ray Photoelectron Spectroscopy (XPS) was used to testthe nitrogen binding energy. FIG. 26 includes two line diagramsillustrating nitrogen binding energies before and after the annealingtreatments. The binding energy of N¹⁵ before annealing is about 401 eV,indicating a positively charged state, as compared with a neutral state.This indicates that nitrogen remains its ionized (N+) state afterimplantation. The binding energy of nitrogen shifted to about 397 eVafter annealing, corresponding to a negatively charged state. Thissuggests that nitrogen has combined with iron after annealing.

Example 9

Iron nitride samples were prepared using a cold crucible technique byexposing a strained iron sample to urea, as described with respect toFIGS. 15-18. The iron sample was heated to a temperature of about 660°C. for about 4 hours while being exposed to urea (1 gram urea for eachgram of iron), followed by water-cooling the iron sample.

The iron sample then was cut into wires and stretched using an apparatussimilar to that shown in FIG. 11. Three wires were strained to differentlengths, with the strain being determined using a strain gauge. Thefirst sample was subjected to a tensile force of about 830 Newtons (N),which resulted in a strain of about 2.5%. The second sample wassubjected to a tensile force of about 1328 N, which resulted in a strainof about 4%. The third sample was subjected to a tensile force of about1660 N, which resulted in a strain of about 5%. Each sample was annealedat about 150° C. for about 20 hours while being strained the statedamount.

The crystal structure of the three samples then was characterized usinga Siemens D5005 X-ray diffractometer (XRD) with Cu Kα radiation source.FIGS. 27-29 illustrate XRD patterns collected for the three iron nitridesamples. FIG. 27 illustrates the results for the sample strained to2.5%. The XRD patterns include peaks for Fe₁₆N₂ (002), Fe₁₆N₂ (301), andFe₁₆N₂ (004), along with other iron nitride (Fe₄N (111)), iron (Fe(110), Fe (200), and Fe (211)), and iron oxide (Fe₃O₄) phases.

FIG. 28 illustrates the results for the sample strained to 4%. The XRDpatterns include peaks for Fe₁₆N₂ (002), Fe₁₆N₂ (301), and Fe₁₆N₂ (004),along with other iron nitride (Fe₄N (111)), iron (Fe (110), Fe (200),and Fe (211)), and iron oxide (Fe₃O₄) phases.

FIG. 29 illustrates the results for the sample strained to 5%. The XRDpatterns include peaks for Fe₁₆N₂ (002) and Fe₁₆N₂ (004), along withother iron nitride (Fe₄N (111)) and iron (Fe (200) and Fe (211)) phases.

FIGS. 30-32 are hysteresis loops of magnetization versus coercivity forexamples of iron nitride wires exposed to different strains (2.5%, 4%,and 5%) during annealing. The hysteresis loops were measured at roomtemperature. FIG. 30 illustrates that, after annealing, the samplesubjected to a strain of about 2.5% has a saturation magnetization(M_(s)) of about 138 emu/g. and a coercivity (H_(c)) of about 660 Oe.FIG. 31 illustrates that, after annealing, the sample subjected to astrain of about 4% has a saturation magnetization (Ms) of about 165emu/g. and a coercivity (Hc) of about 750 Oe. FIG. 32 illustrates that,after annealing, the sample subjected to a strain of about 4% has asaturation magnetization (Ms) of about 170 emu/g. and a coercivity (Hc)of about 800 Oe.

Example 10

Pure (110) iron foils with 500 nm thickness were positioned onmirror-polished (111) Si substrates. The surfaces of the substrates andiron foils were cleaned beforehand. The foils were directly bonded withthe substrate by using a wafer bonder in fusion mode (SB6, Karl SussWafer Bonder) at 450° C. for 30 minutes. FIG. 33 is an example infraredimage illustrating an iron foil direct bonded to a (111) Si substrate.

Ions of atomic N⁺ were accelerated to 100 keV and implanted into theiron foils vertically with fluences ranging from 2×10¹⁶/cm² to5×10¹⁷/cm² at room temperature. The samples were exposed to the sameimplant energy but different nitrogen fluencies. After that, a two-steppost-annealing process was applied on the implanted foils. The firstannealing step was pre-annealing at 500° C. in a N₂ and Ar mixedatmosphere for 30 minutes. The pre-annealing was followed withpost-annealing at 150° C. for 40 hours in vacuum.

FIG. 34 is a diagram illustrating example nitrogen depth profiles forthe ion implanted sample before the two-step annealing, which weremeasured by Auger Electron Spectroscopy (AES) with Ar⁺ as the in-depthmilling source. FIG. 34 shows the nitrogen distribution before annealingfor four samples with implantation fluencies of 2×10¹⁶/cm², 8×10¹¹⁶/cm²,1×10¹⁷/cm² and 5×10¹⁷/cm², respectively. The nitrogen implant rangeinside the foil is determined by the implant energy. As shown in FIG. 1,the four samples have substantially the same implant range (about 160nm) and the substantially the same peak position (about 100 nm). This iscoincident with the simulation result by SRIM.

FIG. 35 is a diagram illustrating example nitrogen depth profiles afterthe two-step annealing. Nitrogen concentrations in the foils aresubstantially homogeneously distributed throughout the depth of thefoils after annealing. The nitrogen concentration distributioncorresponding to 1×10¹⁷/cm² fluence is about 11 at. %, close to thestoichiometric ratio of nitrogen in Fe₁₆N₂. For the sample with5×10¹⁷/cm² fluence, the nitrogen concentration has surpassed theachievable maximum nitrogen solubility in iron.

The crystal structure of foil samples was characterized using SiemensD5005 X-ray diffractometer (XRD) with Cu Ka radiation source. FIG. 36 isa diagram illustrating example XRD spectra for foil samples withdifferent nitrogen fluencies on Si (111) substrate after post-annealing.Fe₁₆N₂ phase always exists at tested all conditions with ion fluencesvaried from 2×10¹⁶/cm² to 5×10¹⁷/cm². For ion fluences of 2×10¹⁶/cm² and8×10¹⁶/cm², only the Fe₁₆N₂ phase iron nitride was observed. For thesamples with fluences of 1×10¹⁷/cm² and 5×10¹⁷/cm², because the nitrogenconcentrations were larger than 11.1 at. %, the ε-iron nitride (Fe₄N)peak was also observed. The FeSi (111) phase can be observed at all thefluences, indicating an iron silicide interface appearing due to fusionbonding at 450° C.

FIG. 37 is a diagram illustrating example hysteresis loops of the sampleprepared with 1×10¹⁷/cm² fluence at different stages, which weremeasured in the foil plane using a Vibrating Sample Magnetometer (VSM)calibrated by a standard Ni sample at room temperature. The three M-Hloops correspond to the same sample before ion-implantation, afterion-implantation and a pre-annealing step, and after ion-implantationand after both pre-annealing and post-annealing steps. For the pure ironfoil before ion-implantation, its magnetic property is in good agreementwith Fe (110) single crystal. Its remanent magnetization value is equalto its saturation magnetization value with 4πM_(S), which is around 2.02T at room temperature. For the sample after the ion-implantation and500° C., 0.5 hr pre-annealing step, the saturation magnetizationincreases about 7%, up to 2.15 T. Meanwhile, the remanent magnetizationvalue after the pre-annealing step was reduced and the saturation field(Hs) was increased up to about 1000 Oe, which indicates the existence ofthe Fe₈N phase after the pre-annealing step.

The post-annealing step at 150° C. for 40 hours tremendously changed theMH loop of the sample, which matches well with the formation of theFe₁₆N₂ phase in the sample as indicated in FIG. 36. As shown by in FIG.37, a hard magnetic behavior is clearly observed with a saturation fieldof 5.3 kOe. This is consistent with the existence of largemagnetocrystalline anisotropy due to the body-center-tetragonal (bct)structure in Fe₁₆N₂. Additionally, a 15% increase in the saturationmagnetization is observed in this sample, which is greater than the VSMtesting error. The absolute M_(s) value is increased to 2.32 T, comparedto 2.02 T for the control sample (the starting single crystal ironfoil). XRD pattern as shown in FIG. 36 presents clearly the mixed phasesof Fe and Fe₁₆N₂ in the sample. The estimated volume ratio for theFe₁₆N₂ phase in this sample based on the XRD pattern is about 35%. Thus,the saturation magnetization of Fe₁₆N₂ phase in this sample iscalculated as about 2.9 T. For this calculation, we assumed that the Fematrix in the sample possesses the same saturation magnetization asbefore the ion-implantation and annealing, which is reasonable since weused the same sample before and after ion-implantation.

For the sample with 5×10¹⁷/cm² fluence, its magnetic property beforeannealing is substantially similar to the result shown in FIG. 37.However, the magnetic properties of the film change significantly afterthe post-annealing, as shown in FIGS. 38 and 39, with an obviousincrement in coercivity and so, the maximum energy product. FIG. 38 is adiagram illustrating an example hysteresis loop of the sample preparedwith 5×10¹⁷/cm² fluence after post-annealing. The coercivity of the filmis 1910 Oe, the saturation magnetization is 245 emu/g, and the energyproduct ((BH)_(max)) is about 20 MGOe. FIG. 39 is a diagram illustratingthe calculated energy product of the film tested to obtain the resultspresented in FIG. 38. FIG. 39 illustrates that the maximum value of theenergy product of the film is about 20 MGOe.

FIG. 40 is an example high resolution transmission electron microscopy(HRTEM) image of the film tested to obtain the results presented inFIGS. 38 and 39. The TEM sample was obtained by cutting and polishingthe foil perpendicular to the surface by Focused Ion Beam (FIB) (FEI™Quanta 200 3D, available from FEI™, Hillsboro, Oreg.). As shown in FIG.40, the sample is consisted of granular structure. Sub-regions of thesample correspond different crystal structures, one with lattice fringedistance of 2.022 nm corresponding to α″-Fe₁₆N₂ (220), the other withlattice fringe distances of 1.897 nm and 2.027 nm corresponding to Fe₄N(200) and Fe (110), respectively. FIG. 41 is an example image showing anx-ray diffraction pattern for the sample tested to obtain the resultspresented in FIG. 40.

Various examples have been described. These and other examples arewithin the scope of the following claims.

The invention claimed is:
 1. A bulk permanent magnetic materialcomprising: Fe₁₆N₂ phase domains, wherein the Fe₁₆N₂ phase domains formhard magnetic domains; a plurality of nonmagnetic atoms or moleculesforming domain wall pinning sites; and soft magnetic material comprisingFe₈N, wherein at least some of the soft magnetic material ismagnetically coupled to the hard magnetic Fe₁₆N₂ phase domains.
 2. Thebulk permanent magnetic material of claim 1, further comprising: dopantelements to form domain wall pinning sites within the magnetic material.3. The bulk permanent magnetic material of claim 2, wherein the dopantelements comprises at least one of cobalt (Co), titanium (Ti), copper(Cu) and zinc (Zn).
 4. The bulk permanent magnetic material of claim 1,wherein crystallographic textures of the Fe₁₆N₂ and the Fe₈N arecoherent.
 5. The bulk permanent magnetic material of claim 1, whereinthe Fe₁₆N₂ phase domains are chemically ordered.
 6. The bulk permanentmagnetic material of claim 5, wherein the Fe₈N comprises α′ phase Fe₈N,wherein the Fe₈N is a chemically disordered counterpart of thechemically-ordered Fe₁₆N₂ phase domains.
 7. A method of producing thebulk permanent magnetic material according to claim 1, comprisingpositioning an iron workpiece on a surface of a substrate; carrying oution implantation to implant N+ ions in an iron workpiece; andsubsequently performing two annealing processes, pre-annealing andpost-annealing, wherein the pre-annealing is carried out to attach theiron nitride workpiece to the substrate; and wherein the post-annealingis carried out at a temperature lower than a pre-annealing temperature.8. The method of claim 7, further comprising: facilitatingtransformation of a crystalline structure of at least some of crystalsin the iron workpiece from body centered cubic (bcc) iron to bodycentered tetragonal (bct) iron nitride during the post-annealing; andforming Fe₁₆N₂ and Fe₈N phases.
 9. The method of claim 7, wherein thepre-annealing is carried out at a temperature in a range of from 450° C.to 550° C.
 10. The method of claim 7, further comprising: applying anexternal force between about 0.2 gigapascals (GPa) and about 10 GPabetween the iron workpiece and the substrate during the pre-annealing.11. The method of claim 7, wherein an atmosphere in which thepre-annealing step is performed comprises at least one of nitrogen,argon, and hydrogen.
 12. The method of claim 7, wherein the temperaturefor the post-annealing is below 250° C.
 13. The method of claim 8,further comprising: combining multiple workpieces of the Fe₁₆N₂+Fe₈N,and pressing the combined multiple workpieces together to form the bulkpermanent magnetic material.
 14. The method of claim 13, furthercomprising: introducing magnetically soft or nonmagnetic dopantmaterials.
 15. The method of claim 7, wherein the substrate comprises atleast one of silicon and gallium arsenide (GaAs).